Sony A7R4 (61 MP) vs Agfa APX100 (B&W-Film) – Analog vs Digital comparison

This article describes, how I made the resolution-power of lenses digitally measurable on analog film  and COMPARABLE to the data, which are directly measured on digital sensors – using the same algorithm: IMATEST.

Since a long time I am looking for an experimental set-up, which allows me to understand, how the information content of the exposure on an analog film compares to the digital data from a digital sensor – looking through the same lens. Resolution being the main point of interest for me in this case.

Just to give you a quick impression of my results I show here the resolution charts from IMATEST on B&W-film (Agfa APX100) and on Sony A7R4 (61 MP), using the same Olympus SLR-lens OM 28mm f/2.8 (introduced 1973) – (the method will be explained in detail further down in this article):

Fig. 1: Resolution-chart, generated with Olympus OM Zuiko Auto-W 28mm f/2.8 lens on black and white negative film (Agfa APX100) and filmscanner reflecta RPS 10M – MTF30-resolution-values from center to corner for all apertures – source: fotosaurier

I do not think, that these are the „real“ limiting MTF30 resolutions values of the lens. These may be definitely higher – especially in the range betweenf f/5.6 and f/16. For me the purpose of the method is, to clarify the behavior of many (legendary!) historical lenses which show very low resolution values especially in the corners and at stop-down values of f/16 or f/22.

Let us take a look at the digital picture, taken with the Sony A7R4:

Ima_GRAPH_OM28f2,8_A7R4
Fig. 2: Resolution-chart, generated with Olympus OM Zuiko Auto-W 28mm f/2.8 lens on 62 MP-Sensor of Sony A7R4 – MTF30-resolution-values from center to corner for all apertures – source: fotosaurier

Do not let you confuse by the blue lines on different levels, which represent the Nyquist-Frequency in each set-up: the Sony’s sensor has a Nyquist Frequency of 3.168 LP/PH (linepairs per picture hight) – the filmscanner which was used to digitize the analog picture (reflecta RPS 10M) was used at its max. resolution of 5.000 ppi – that corresponds to 2.383 LP/PH as a Nyquist Frequency and delivers ca. 33 MegaPixel pictures.

There is no affordable filmscanner with higher resolution on the market!

This means: the Nyquist Frequency of the Sony Digicam is exactly 25% higher than that of the scanner, which we used as a A/D-converter for the B+W-negatives on the APX100-film.

The highest resolution in the film-based pictures generated with the analog-digital data-processing chain in Fig. 1 is very close to or above the Nyquist Frequency of the scanner – and over the full format area of 24mm x 36mm the resolution in the analog film is gathering very closely under or around this Nyquist Frequency at nearly all apertures, with the exception of open aperture f/2.8 where it is 10-20% lower.

In contrary to that, in the digital pictures taken with the Sony Sensor (Nyquist Frequency: 3.168 LP/PH) the resolutions vary strongly between corners and center and in between (part way) – and for the different apertures.

Let’s look at the center-values of resolution (green curves in Fig 1 + 2): between f/2.8 and f/11 the analog and digital values develop quite constant around the respective Nyquist Frequency, which explains, that the center values on film are 25% lower than on the 62 MP-sensor. But: The drop-off in resolution at f/16 and f/22 on the digital sensor is dramatical and shows that it is a sensor-created artefact.

Looking at the grey curves in Figs 1 + 2: „part way“ between center and corner represents the biggest area of the picture, dominating the perception of the picture! Here the MFT 30 resolution values are higher on film at nearly all apertures in spite of the lower Nyquist Frequency.

The most dramatical difference between analog and digital pictures, however, is – as expected! – in the corners (yellow curves on Figs 1 + 2):

For a better understanding I put the corner-resolution of film and sensor together in one graph:

Fig. 3: Olympus OM 28mm f/2.8 corner resolution on Sony A7R4 (yellow curve) and b+w-film APX100 (grey curve) – source: fotosaurier

The corner-resolution on the sensor with 25% higher Nyquist Frequency starts at f/2.8 at 50% of that of the analog film, exceeds the absolute analog value at f/8, peaks at f/11 with 82% of the sensors Nyquist and drops below the analog-value at f/22, whereas the analog-resolution on film reaches 95% of Nyquist at f/5.6 and stays at about 90% until f/22.

What the resolution-graphs here clearly show: also the very low resolutions in the corners (and even part-way!) of the digital sensor (especially open aperture!) are an artefact of the sensor! We know, that most of the effect is caused by the thick filter stack in front of the sensor. With this picture we know, that this happens not only with rangefinder-lenses, where the corners are literally BLURRED on the sensor – but also with SLR-lenses as in this case! With rangefinder-lenses the difference in corner resolution between analog (film) and digital (sensor) may be 6 to 7 … whereas with SLR-lenses I experience values of 2 to 3.

I confirm again: it is the identical lens in both cases! And these results are pretty much representative for many analog lenses! I will supply you with the results of many more lenses soon. There is one (rangefinder-)lens already analysed with the same method (link here).

EXPLAINING the Method in detail:

1. Extending the digital IMATEST lens testing method and software to pictures taken on analog film:

A. Measuring the optical performance on a digital sensor is facing several facts and influences, which are new and specific: pixel size, algorithm, problems of digital signal-processing systems like aliasing, additional optical elements in the optical path like filter stacks and micro-lenses!

The question: is there an essential influence of all these optical systems on the visual result in the picture over the picture-circle (Bildkreis), e.g. because of the varying angles at which the light-rays hit on the sensors between center and the farthest corner of the picture format or due to the additional optical elements introduced into the light-path?

In the case of RANGEFINDER-lenses we know, that there often is a strong influence of this. These lenses are often made for a very short distances between the last lens and the film – especially for wideangle- and standard-lenses. Little was known to me about historical SLR-lenses, which were never planned and calculated for the use with modern digital sensors. The degradation of the picture quality in the corners of rangefinder-wideangle-lenses is so dramatical, that it is clearly seen, that this is an artefact of the sensor, because we see sharp corners on film with the same lens.

Since several years I do quite a few measurements on historical lenses, using a high-resolution digital sensor with 62 Mega-Pixels, resulting in 60,2 MP effectively on Full Format (35mm stills).

Until now I did not know, whether the measurement of my historical SLR-lens is falsified due to artefacts, generated by the digital recording system. The work, described in this article, was done, to clearify this situation.

I just want to know: how does picture quality of historical SLR-lenses on the analog film compare measurably to that delivered by digital sensors?

Digital cameras are really big number-crunching-machines! And with the right software, I can use the numbers to generate a numerical picture of  the optical quality of the lens-sensor-combination. IMATEST is such a software and it uses standardised TARGETS to do that. I use the following target:

DSC03033_Macr-Yashica_55f2,8_5,6-foc Kopie
Fig 5: SFRplus target for Imatest – it’s height is 783 mm between the horizontal black bars, which means, that the reproduction ratio on film is 33:1 – source: fotosaurier/Imatest – original information graphics from IMATEST

Over years I did – like many other amateur-photographers – compare real-world photos of analog vs. digital processing. But I was never satisfied, because this method gave me only subjective impressions – it did not create reproducible figures, to generate a precise description of the results!

I collected intensive experience with IMATEST on more than 150 lenses over meanwhile 5-6 years using the digital pictures generated by digital sensors (4,9 to 102 Megapixels) of seven different DIGICAMS. During this time, my Standard Digicam to compare lenses was (and still is) Sony A7R4 (62 Megapixels) – since it had arrived in the market (2018/19).

IMATEST (Studio) software delivers MTF-based resolution data – as it can do that separately in three RGB-channels, it also delivers lateral CA-data. Using the Target structure of Fig. 5, the software selects 46 local areas, and runs the MTF-measurement automatically for all these 46 areas. The following picture demonstrates the automatic areas, which are typically selected – but you could choose others as well:

ROI-chart (standard)
Fig. 6: The 46 magenta rectangles (called „ROI„) frame the Edges in the target, at which the 46 MTF-measurements are made – source: fotosaurier/Imatest

These are the curves, which are generated from each digital picture (black&white):

Zusammenstellung_IMATEST_A7R4_OM28d2,8_2,8
Fig. 7: Summary of the  IMATEST-results for the OM28mm f/2.8 at open aperture f/2.8 on Sony 62 MP-sensor (A7R4) – explanation see text beneath – source: fotosaurier

The upper left curve shows the edge-profile at center of the target (ROI no. 1, which is the left (vertical) edge of the center square in Fig. 6). From this graph the edge-rise between 10% and 90% is taken from the x-coordinate in pixels. The lower left curve is the MTF-curve (contrast over spatial frequency) for the same location. From this graph the MTF30 value (Frequency at 30% contrast) is taken: follow the horizontal line at 0,3 MTF-value to its section with the curve and take the frequency on the abscissa. The right curve shows the MTF30-values of ALL 46 ROIs plotted over the distance from the center in the 35mm-fframe.

I have resumed the IMATEST test-method in more detail in this article here in my blog!

B. Digital measurement of resolution on analog film

Now I decided to make the following experiment:

  • Take a photograph of the IMATEST-target on analog film;
  • digitize the picture with a film-scanner;
  • analyse the resulting digital picture with IMATEST.

For the tests, which I describe here, I used the following hardware:

28mmf2,8-on-OM4Ti_DSCF1655_blog
Fig. 8: Analog SLR Olympus OM-4 Ti with Zuiko Auto-W 28mm f/2.8, loaded with „fresh“ Agfa APX100
  • Camera for the shooting on analog-film: Olympus OM-4Ti
  • Lens: Olympus Zuiko Auto-W 28mm f/2.8 (Ser.no. 232073)
  • Film: B&W negative film AgfaPhoto APX100, iso125, developed in Rodinal 1+25 (8′)
  • Scanner: reflecta RPS 10M film scanner

The OM-4Ti (about 25 years old) and the lens (nearly 50 years old) work still perfect. I let the OM-4Ti automatically generate the exposure time: from 0.4 seconds to 1/250 seconds. The densitiy of the negatives was pretty constant on the film-strip! I use a sturdy tripod, which is made for use with long astronomical telescopes.

With this method I hope to use the full analyzing-power of IMATEST-software on a picture-frame, which is generated through the lens WITHOUT the typical artefacts, which digital sensors MAY generate in the optical path of a historical lens.

ON THE FILM we have now the IMATEST target-pattern, which allows to make a fast and powerfull analysis of optical data over the full picture frame – also very close to the edges and into the corners. This pattern is superimposed by the typical grain-structure of the light sensitive layer – and potential light-diffusion-effects within the film thickness. Both (analog) effects LIMIT the resolution, which can be achieved on FILM.

My first and major interest was always focused on the observation of the enormous difference between the center-resolution (see Fig. 7), which is digitally measured on A7R4 with ca. 3,000 LP/PH or higher) and corner-resolutions of <200 to 600 LP/PH on the sensor .

The question is: are the low values on edges and in coners of the frame, measured with the digital sensors, an artefact, caused by the different light-path? We know definitely about these effects with rangefinder-lenses, which have a very short back-distance between last lens and film, causing big trouble on sensors of mirrorless cameras. This is today well known, to be mainly caused by the thick filter-stacks in front of the sensors (creating field-curvature and cromatic aberrations with analog lenses).

It has been shown, that this can partly be „cured“ – or at least reduced – by reducing or deleting the filter-stack, and/or putting a positive lens (so-called „PCX-filter“) in front of the lens-sensor-combination.

The 35mm-negative-film:

I made my first attempts to photograph the IMATEST-target on film with

  • b&w-film Agfa APX100, iso 100

which is still available as „fresh“ product. For this first step I decided to stay with b&w-film, because I can process it myself under controlled conditions. With colour negative film I would have an external influence, which I could not control! Just for resolution this means no restriction in the information, because CA-errors also blurr the B&W-picture!

I did the devellopment of the b&w-film myself with Rodinal.

The A/D-converting:

The negatives were digitized through my film-scanner reflecta RPS 10M,which offers a maximum linear resolution of 10,000 pixel per inch (PPI).

To me, this step seemed to be very important: to avoid new artefacts from the digitizing algorithm. So I chose a spatial frequency, which is higher than the expected limiting spatial frequency of the film: I set the scanner at 5,000 ppi. On pixel-level this corresponds to an imaging-sensor of ca. 33.7 MP (for 24mm x 36mm).

From my earlier estimations I had found, that a normal recording film for general imaging purposes should correspond to a digital FullFormat-sensor with 20-12 MP.

The picture height, which the scanner digitally delivers (24mm minus a bit of crop to frame the target safely), was 4,676 pixels and so the „Nyquist Frequency“ of the scanner set-up corresponds to 2,338 LP/PH – corresponding to an effective sensor-size of 32,7 Mpxls.

Fig. 7 shows the b&w-picture, which was generated with the scanner:

AGFA100_OM28f2,8_2,8_H4536
Fig. 7: Scanner-output from the b&w negative-film Agfa APX100 from Olympus OM 28mm f/2.8 at full aperture f/2.8. Picture-hight of this original scan is 4.676 Pxls. You see, that the light-fall-off of this lens into the corners is very moderate … and the linear distortion with exactly 1% acceptable as well! – source: fotosaurier

Let’s have a closer look into the structure of this image – in Fig. 7a you get an impression of the grain structure of the films emulsion at about 200% enlargement of the 33 MP-image:

Enlargement-Film-200%
Fig. 7a: Overview of the grain-structure at ca. 200% enlagement of original scan in Fig. 7. The pixel-size here is 5,3 µm – the grains of the film are bigger than the pixels – source: fotosaurier

Following picture is the MTF-curve of the analog image „as scanned“ (in the center of frame):

Fig 8: MTF-curve in. Center (ROI no.1) of OM-Zuiko 28mm f/2.8 at f/2.8 – source: fotosaurier

The „noise“ in the curve is caused by the film-grain, which is about the same size as pixels.

Film_3024-pixel-height_at-800%
Fig. 9: Here we look at about 1,000% into the pixel-structure of the scanned image. At the edges of the dark rectangle (where the resolution is analysed!) the grain-diameter is about the same size. Only some local „grain-clusters“ are considerably bigger – source: fotosaurier

Previous trials had shown, that with a film with this grain-structure, this digital image-size would give adequate results for MTF and resolution.

In the case of a digital sensor of a digicam I avoided generally to use RAW-data, which would have urged me to use my own very personal „development-parameters“ in Lightroom or other software to generate the final picture. I use OOC-JPEG-Data at „Standard“-settings, due to generate conditions (all important parameters set to „zero“), which are transparent and reproducible for everybody with the same camera-model! That means: it would also have been possible to create pictures with much higher resolution results in Imatest, e.g. by setting higher sharpening-parameters or the „clear“-mode.

Now with a film-scanner I had to go myself through a very intensive process of defining the „development-parameters“ in Silverfast. Starting with the setting to 5.000 ppi for the basic scan-resolution. With 10.000 ppi, which is offered with this model, you will get no REAL increase in EFFECTIVE resolution.

However, using the „Multiple Scan Mode“, you extend the accessible resolutions above the „Nyquist Frequency“, which would be 2.383 LP/PH, corresponding to a Picture size of 32,7 MP

My target was, to reach about the same level of resolution in the center of the scanned images on analog film as with the Sony A7R4 images, which means in the range of 3.168 LP/PH, which is the Nyquist Frequency of the Sony Sensor.

This corresponds with a resolution of 260 Lines/mm.

I came close to this with the following settings:

Fig. 10: Scan-parameters in Silverfast 8 on film-scanner RPS 10M – source: fotosaurier

See the complete results here:


Fig. 11: Analog on film resolution results of Olympus OM 28mm f/2.8 SLR-lens with b+w-film APX100, scanned with RPS 10M film-scanner – source: fotosaurier

The interpretation of this in comparison with the measurement-results on the 62 MP-sensor of the Sony A7R4 (Fif. 2) has been given in the first section of the Article.

Finally I asked myself, whether a PCX-filter (lens) could improve the resolution-artefacts which are found on the sensor? But I found no real positive effect.

Fig. 12: Resolution of OM 28mm f/2.8 lens with PCX-3m lens on the Sony A7R4-sensor: no improvement at all – source fotosaurier
Fig. 13: Soon I will enter a new article, showing the performance of this wideangle-lens on seven different cameras – link not yet available … stay tuned!

Copyright „fotosaurier“

Herbert Börger, Berlin, November 2023

Two crazy lenses of the 1950s – Angénieux 50mm f/0.95 and Carl Zeiss Jena Biotar 50mm f/1.4 for 35mm Cine-Format – plus Canon Lens 50mm f/0.95 from end of 60s

A few weeks ago I was blessed, having an Angénieux 50mm f/0,95-lens and a „Biotar 50mm f/1.4″, at the same time in the same place !

An Angénieux 50mm f/0,95-lens in perfect optical quality and with aperture-mechanism  and rehoused into a perfect Sony-E-body, focusing to infinity and ready for measurement in my optical IMATEST-Lab…. this is really a „unicorn“!


Fig. 1: Ultra-rare 50mm f/0.95-lens for Cine 35 movie-format – this lens-series (10mm, 25mm and 50mm) founded Pierre Angénieux‘ high reputation in cinematic optics! – source: fotosaurier

The „Biotar 50mm f/1.4″, in great overall condition, which I even did no know about, before I saw it for the first time.

Biotar58f1,4-front_DSCF1765
Fig. 2: One of the best high-speed-lenses ever made in Jena – Biotar 50mm f/1.4 of 1955/56 for Pentaxflex AK-16 cine-camera system – professional performance for professional use! – source: fotosaurier

Photo-friend and co-nerd Thomas handed out both ultra-rare lenses to me for closer optical inspection. I am a happy man!

Fig. 3: Two very rare lenses at the same time in the same place … in my IMATEST-Lab! Sheer happiness! – Source: fotosaurier

  1. Angénieux 50mm f/0,95 (Type M1):

Thomas has proven, that it is possible to re-house the Angénieux-lens for general photographic use with infinity focus:

Fig. 4: The early super-fast Angénieux 50mm f/0.95 lens 0f 1954/55 here in a „Unikat„-version – the basic lens is directly fitted to E-Mount for Sony – source: fotosaurier

Starting in 1953 Pierre Angénieux brought out a series of lenses with f/0.95. In 1953 it was firstly the 25mm f/0.95 (which became the most famous Angénieux lens due to the use in NASA-spaceflights to the moon!) made for cine 16mm format and the 10mm f/0.95 for 8mm-cine.

A few months later he pushed out also a version for 35mm-cine: the 50mm f/0.95 – probably this was in in 1954 – originally in C-Mount. Hartmut Thiele dates this to 1955. It is important to understand, that this is not a lens made for still-photogray amateur use – but Pierre Angénieux showed here all his knowledge dedicated for professional cine-use. He went to the limits of everything, which was possible with glass-types and design- and production-methods at that time!

If you need more information on Pierre Angénieux, please look up my Blog article here!

Following my measurements on the IMATEST-target the picture-circle, that this lens covers is 37mm – so it is falling a bit short from the 43mm needed for covering the still-photo-35mm-full-format (24 x 36 mm).

DSC05014_Ang_50f0,95_0,95-foc1,4_Bildkreis
Fig. 5: Picture of IMATEST-Target through Angénieux 50mm f/0.95 at f/0.95 in the 24 x 36 mm full-frame of the Sony A7R4 – Source: fotosaurier

This test-set-up generates the following resolution-measurement results:

Fig. 6: Resolution at center/part way/corner of Angénieux 50mm f/0.95 on Sony A7R4 (60,2 MP-sensor – 9.504 x 6.336 pixels!) at standard distance full-frame (24×36) – Source: fotosaurier

In spite of the heavy darkening in the corners, the system does still generate results, but these readings are not very reproducible … these corner-readings are located clearly outside the picture-circle for this lens!

So I made a second set-up with the camera set a little bit further away from the target, so that the individual measuring areas move somewhat towards the center of the picture and do not suffer too much from the dark areas out of the picture circle of the lens.

Fig. 7: Angénieux 50mm f/0.95 moved a bit backwards from the target – measurement-areas (marked magenta rectangles) moved somewhat further towards the picture center – avoiding overlap with the dark corners – this picture is at f/8, showing a sharper limit to the dark corner-areas! – source: fotosaurier

Now the furthest measurement locations are at 82% of the full-frame picture radius, clearly inside the bright circle which this lens covers at 86% of full-frame radius!

The result is seen in the following picture:

Fig. 8: Resolution with refocussed Angénieux lens 50mm f/0.95. The corner-resolution-values are still located outside the Cine35-picture-frame!!! The „peak“ at f/4 in the corner reading is real – no error – never seen anything like this with any other lens! – source: fotosaurier.

In Chapter 4 at the end of the article I will ad thwe measuremts at cine-format for all three lenses (Super 35: 18,66mm x 24,89mm). This will give more realistic resolution-readings. The Super35 crop-mode on the A7R4 is  6.240 x 4.160 pixels.

2. Carl Zeiss Jena Biotar 50mm f/1.4:

About the same time, DDR-based Carl Zeiss Jena created a high-speed lens for its own Pentaxflex AK-16 cine-camera system in Pentaflex-16 mount.

It seemed logical to follow the already successfull BIOTAR-formula and it came out around 1955 or 1956 the Biotar 50mm f/1.4:

Biotar58f1,4-2_DSCF1757
Fig. 9: Carl Zeiss Jena Biotar 50mm f/1.4 for Cine-Format, arriving 1955/56 – Source: fotosaurier

Looked at with the sensor of the Sony A7R4, the picture-circle is a bit larger than with the Angénieux … there are only minimal dark corners!

Bildkreis_DSC05072_Biotar-50f1,4_1,4-just-foc
Fig. 10: Full-frame picture of IMATEST-target through Biotar 50mm f/1.4 at f/1.4 – Source: fotosaurier

Of course, we have here the same situation, that the corner-measurements are quite a bit outside the cine-picture frame of typically 16mm x 22mm:

Biotar_50f1,4_FF_Graph

Fig. 11: Biotar 50mm f/1.4 in the same frame as Angénieux seen in Fig. 7 – source: fotosaurier

I will also with this lens repeat the measurement, restricting the resolution-target to the cine-picture frame – see section 4 at the end of the article.

The results show for both lenses, that the resolution in the center is extremely high – even wide-open! Both lenses are extraordinary lenses of their time – the mid-1950s!!!

Unique: „first-in-industry“ point of view for the Angénieux 50mm f/0.95 in its extreme speed, without sacrifycing to the center resolution!

3. Canon Lens 50mm f/0.95 for rangefinder (Canon7) cameras with LTM 39mm – of 1969

As we are just talking about early historical high-speed lenses, the step to the famous CANON 50mm f/0.95 (for rangefinder) is logical. It is a step of 15 years in time – and this time the lens is really dedicated to 35mm still-photo full-format 24mm x 36mm!

Noch'nPaar_DSCF1775
Fig. 12: Angénieux 50mm f/0,95 of 1954, left, and Canon 50mm f/0.95 of 1969 / the normal still-photo-version here – Source: fotosaurier

Here is my comparable resolution-measurement with two samples of (s.n.18924 and s.n.22372) of the Canon 50mm f/0.95 on Sony A7R4 for this lens at full 24×36-format:

Crf_50f0,95_Graph
Fig. 13: Resolution-Graph of Canon 50mm f/0.95 on Sony A7R4 (60,2 MP) – Source: fotosaurier

Fig. 13a: Resolution-Graph of Canon 50mm f/0.95 on Sony A7R4 (60,2 MP) – Source: fotosaurier

To allow for the necessary rangefinder-coupling besides the huge rear lens, this lens is „cut free“ at the edge for this purpose.

Crf59f0.95_DSCF1687
Fig. 14: Cut-away at the 50f/0.95 Canon’s rear lens, to allow for the rangefinder-coupling! – source: fotosaurier

However, the 50mm f/0.95 lens was also released in a version for video cameras, with an additional engravureTV“ on the nameplate: consequently these lenses were delivered with C-mount. As these lenses do not need the rangefinder-coupling, the rear lens is not cut at the edge here.

Hopefully I wil be able to add a picture of the 50mm f/0.95 TV-lens rear section for comparison soon.

4. Finally: Resolution-Data of these Lenses, measured for the Cine Super35-format, which the Angénieux and CZJ Biotar Lenses are originally dedicated to – on all three lenses:

a) Angénieux 50mm f/0.95 – at Super35-format:

Fig. 16: Angénieux 50mm f/0.95 at Super35-frame on Sony A7R4 – absolutely phantastic for this „first-in-speed“  – source: fotosaurier

b) Biotar 50mm f/1.4 – at Super35-format:

Fig. 17: CZJ Biotar 50mm f/1.4 at Super35-frame on Sony A7R4 – absolutely phantastic for this „first-in-speed“  – source: fotosaurier

c) Canon 50mm f/0.95 – at Super35-format on Sony A7R4:

Canon lens f=50 mm f:0.95_A7R4_Super35_Graph
Fig. 18: Canon Rangefinder 50mm f/0.95 – primarily dedicated to still-photo 24×36 but also delivered as a TV-version – just a bit better than the Angenieux, but 15 years later! – source: fotosaurier

All three lenses have very low chromatic aberrations in the center but the Canon peaks out in maximum CA, Biotar and Canon are close to zero in distortion, while the Angenieux has around -1% distortion, which is still excellent for such an early, extreme lens!

5. Appendix:

Here you see all properties of the three lenses in detail – for 24×36 (full frame) and Super 35 (cine-format).

5-a1. Angenieux M1 50mm f/0.95 – FullFormat 24×36.

5-a2. Angenieux M1 50mm f/0.95 – Super35.

5-b1. Carl Zeiss Jena Biotar 50mm f/1.4 – FullFormat 24×36.

Spreadsheet_Biotar-50f1,4_FF

5-b2. Carl Zeiss Jena Biotar 50mm f/1.4 – Cine35.

Spreadsheet_Biotar-50f1,4_Cine35

5-c1. Canon Rangefinder 50mm f/0.95 – FullFormat 24×36.

Spreadsheet_Crf50f0,95_FF_sn18924

5-c2. Canon Rangefinder 50mm f/0.95 – Cine35.

Spreadsheet_Canon-50f0,95_Cine35

Herbert Börger

Berlin, 24.12.2022

Hey Sony! Was passiert bei der Objektiv-Korrektur in meiner Sony A7Rm4 ?

Bei hochwertigen digitalen Systemkameras hat man üblicherweise die Möglichkeit, eine digitale „Objektiv-Korrektur“ zuzuschalten – für moderne Objektive, deren Eigenschaften in der Firmendatenbank des Kameraherstellers meines Vertrauens gespeichert und für das Kameramodell verfügbar sind. Dazu muss die Kamera das Objektiv-Modell erkennen und die notwendigen Korrektur-Algorithmen besitzen – oder das Objektiv könnte diese Informationen über seine Fehler in sich tragen.

Ich mchte nur generell erwähnen, dass ich in allen meinen Testberichten, in denen ich historische und moderne Objektive verglichen habe, immer die Objektiv-Korrektur ausgeschaltet habe.

Geben Sie es zu: Sie waren bisher auch so naiv, zu glauben, dass da auf wundersamem – eben digitalem! – Wege die aufgrund der bekannten Rest-Fehler der Optik fehlerhaften Bildinformationen „nachgebessert“ werden. Es entstehe bitte: DAS PERFEKTE BILD – bei Verwendung eines un-perfekten (und damit billigeren) Objektives, dessen Rest-Fehler durchaus sehr groß sein könnten – man müsste sie nur kennen …

Nachdem ich persönlich schon relativ sicher war, dass von der „Objektiv-Korrektur“ KEINE WUNDER zu erwarten sein werden, wollte ich mal nachschauen, was denn wirklich passiert. Was können wir heute von einer Objektiv-Korrektur erwarten, wobei ich das Thema erst einmal auf die 60 Megapixel-Sony-Kamera A7Rm4 beschränken muss, also einen aktuellen, hochauflösenden Sensor.

Meine Hoffnung ist, dass beim Aufbereiten der Sensor-Rohdaten diese Kamera nicht schon ohne mein Wissen die Bilddateien manipuliert, solange die Objektiv-Korrektur ausgeschaltet ist! Bei den historischen Objektiven, die ich normalerweise sehr überwiegend analysiere, besteht diese Sorge ja ohnehin nicht, da das Objektiv normalerweise nicht mit der Kamera kommunizieren kann – die Kamera aber auch sowieso nichts über mein „Ernostar“ von 1926 weiß!

Ich sollte nicht verschweigen, dass meine Motivation, diesen Bereich näher zu untersuchen dadurch plötzlich für mich höhere Priorität erlangte, dass ich versucht habe, in Dateien auf Basis des IMATEST-Test-Targets die Vignettierung mittels Photoshop zu kompensieren, um zu erfahren, welchen Einfluß die Vignettierung alleine (also der Helligkeitsabfall zum Rand) auf die Auflösungsmessung haben könnte.

Die erneute Analyse der manipulierten IMATEST-Target-Datei ergab: einen KATASTROPHALEN Einbruch der Auflösungswerte überall im Bild. Das hat mich schon sehr alarmiert!

Zufällig um dieselbe Zeit habe ich mein Referenz-Normalobjektiv (Sony Planar FE 50mm f/1.4 ZA) erneut mit IMATEST gemessen – und erreichte nicht annähernd die mir geläufigen hohen Auflösungs-Werte. Ich sellte fest, dass – durch irgendeinen Zufall – die Objektiv-Korrekturen eingeschaltet waren.

In der Folge führte ich folgendes Messprogramm durch – wobei ich das exzellente (aktuelle) Planar FE 50mm f/1.4 ZA im E-Mount (Sony) verwendete. Nach meinen umfangreichen Erfahrungen kann das verwendete Objektiv aber durchaus als Referenz dessen gelten, was in diesem Preissegment heute möglich ist.

Auflösungs-Messung (mit CA- und Verzeichnungs-Daten sowie Kantenschärfe-Messung) an der Sony Planar FE 50mm f/1.4 ZA am Imatest-Target (SFRplus):

Laborszene900
Bild 1: Messanaordnung Mit Sony A7Rm4-Kamera und dem großen IMATEST-SFRplus-Target. Die Höhe des Targets zwischen den oberen und unteren Balken beträgt 783 mm, Der Abstand mit 50mm-Objektiv ca. 1,6 m.

Beschreibung des Messverfahrens im Detail siehe:

Fotosauriers optisches Testverfahren für Objektive mit IMATEST

Die typischen individuellen Fokussier-Unsicherheiten der (eigentlich überlegenen) Manuellen-Fokussierung wollte ich zunächst vermeiden, deshalb wählte ich Autofokus für die Schärfeeinstellung – und zwar mit Fokusfeld im Zentrum.

Die Objektiv-Korrekturen sind AUSGESCHALTET (OFF):

50f1,4_AF-oKorr
Bild 2: Auflösung, Kantenschärfe und Verzeichnung (IMATEST) mit Autofokus, Objektiv-Korrekturen ausgeschaltet – PLANAR 50mm f/1.4 – gegenwärtiger Stand der Technik (2018)

Anschaulicher sind die folgenden Grafiken, Auflösung (LP/PH = Linienpaare/Bildhöhe) über der Blende aufgetragen – jede Zahl ist ein Mittelwert über mehrere Messpunkte (insgesamt 46 Messpunkte bei jeder Blende über die gesamte Bildfläche verteilt):

FE 50f1,4_Autofocus_oKorr_Diagramm
Bild 3: Diagramm Auflösung (Mitte-Übergang-Ecken) des FE 50f1.4 ZA mit Autofokus

Das folgende Diagramm zeigt die Auflösung desselben Objektivs  mit EINGESCHALTETER VERZEICHNUNGS-KORREKTUR

FE 50f1,4_Autofocus_mVerzKorr_Diagramm
Bild 4: Auflösung (Mitte-Übergang-Ecken) (IMATEST) mit Autofokus, Objektiv-Korrekturen eingeschaltet – PLANAR 50mm f/1.4 – gegenwärtiger Stand der Technik (2018)

Man erkennt sofort, dass die Auflösung in der Bildmitte („Center“ – grüne Linie!) sehr stark abgesunken ist gegenüber der Messung ohne Verzeichnungskorrektur. Wenn man genau in die Rand-Auflösungswerte schaut, sieht man, dass zwischenBlende 2.8 und 8 die Auflösung auch in den Ecken und im Übergang (part way) leicht verringert ist. Außerdem ist die Kantenschärfe in der Bildmitte (Wert „Edge profile bzw. sharpness“) deutlich – nämlich ebenso um ca. 20% wie die Auflösung in Bildmitte – reduziert.

Die Aufgabe der Verzeichnungskorrektur wird dabei allerdings vorbildlich gelöst: die Verzeichnungswerte werden mit 0,03-0,07% auf bis zu ein Zehntel der ursprünglichen Verzeichnung von 0,35% abgesenkt – dann meist mit der Charakteristik „Moustache“.

Die Frage ist nur: zu welchem Preis in der Bilqualität geschieht das hier? Und ist das Objektiv damit sinnvoll verwendet. Mit Listenpreis € 1.500 erstehe ich eine 12-linsige Festbrennweite mit state-of-the-art Optik (Asphäre, Sondergläser). Da möchte ich die volle optische Leistung (schon ab Offenblende!) gerne genießen!

Die oben dargestellte Erkenntnis ist daher wohl von eher theoretischem Interesse. Eine Verzeichnung von 0,35% ist ohnehin so gering, dass sie praktisch nicht auffällt. Man solte den 12-Linser nicht „abdrosseln“ und ihm damit seine optische Potenz nehmen.

Zu der anderen angebotenen Objektiv-Korrektur, die man in der A7Rm4 einzeln zu- und ab-schalten kann, läßt sich allerdings nur Positives sagen: die CA-Korrektur beeinflusst hier die Auflösungswerte allenfalls positiv – nämlich da, wo im Rand-Ecken-Bereich der Farbfehler reduziert wird: dort steigt dann auch die Auflösung. Das Zuschalten ist also auch bei einem derart hoch-korrigierten Objektiv zu empfehlen. Die Wirkung ist auch in der Bildmitte nachweisbar.

Für dieses hier besprochene Objektiv würde ich dringend empfehlen, die Lens-Correction Funktion auszuschalten und lediglich die CA-Korrektur einzeln zuzuschalten.

Bei anderen Hochleisungs-Objektiven habe ich dasselbe überprüft und bin – glücklicherweise – ausschließlich zu anderen Ergebnissen gekommen, wie man in den folgenden Tabellen sieht. Ich habe dabei nur die Performance bei voller Öffnung dargestellt, da die Objektiv-Korrektur da typischerweise am stärksten eingreift.

Hier drei Beispiele mit drei der aktuellsten hochklassigen Optiken mit 40 mm Brennweite ebenfalls an der Sony A7R4:

Batis-40mmf:2.0_with:without-Correct_0penApert
Bild 5: Auflösung, Verzeichnung und CA bei voller Öffnung am Batis 40mm f/2.0 – ohne und mit Lens-Correction – Quelle: fotosaurier

Sony FE40f2,5 - with:without-Correct_openApert
Bild 6: Auflösung, Verzeichnung und CA bei voller Öffnung am Sony FE 40mm f/2.5 – ohne und mit Lens-Correction – Quelle: fotosaurier

SigmaArt-40mmf:1.4_with:withoutCorrect_openApert
Bild 7: Auflösung, Verzeichnung und CA bei voller Öffnung am SigmaArt 40mm f/1,4 – ohne und mit Lens-Correction – Quelle: fotosaurier

Diese drei Beispiele nähren bei mir die Hoffnung, dass die Situation beim Planar 50mm f/1.4 eine Ausnahme sein könnte. In allen drei Fällen zeigt sich grundsätzlich sowohl eine Verbesserung der Verzeichnung als auch der Auflösung, die vermutlich unmittelbar auf die nachträgliche Korrektur der Chromatischen Aberration zurück geht.

Herbert Börger

Der Brandenburger Tor, Berlin, 11. Dezember 2022

My Crazy Lenses / Meine sehr speziellen Objektive – Focal length 24mm / Brennweite 24mm – FoV 84° – Part I

What was the real improvement in SLR-wideangle-lenses since the invention of the retrofocus principle over the last 65 years? Does my personal judgement from analog-film-days which lead to the definition of „legendary optics“ – which I kept in my lens-portefolio over that time – correlate with objective resolution-measurements? Here are my findings.

Actualisation: Im my first published version there was an error regarding the year of appearance of the Topcor 2,5cm-lens, which was communicated to me by a reader: thank you: it’s 1965 instead of 1959! But this difference does not change anything in my findings and conclusions …

1 – Introduction

24mm focal length is a real milestone in spreading the field of the view in wideangle lenses, coming down from FL 35mm over 28mm. For the SLR-camera-user this age started with the appearance of the retrofocus lenses in the 1950s. Several designers came out with this optical principle within three years – with Pierre Angénieux earning the honours of being FIRST (in time and quality – 1950, 35mm f/2.5) in this disciplin.

This is a report about SLR-lenses for 35mm-still-foto-cameras with focal lengths (FL) between 23mm and 25mm.

This is a report about a number of legendary lenses, which I happen to own or could lend from a friend  („phothograf“), most of them being milestones of optical engineering in their respective design-periods.

Drei_24er-Oldies_DSCF1838
Fig 1: three of the very first historical retrofocus-lenses with FL 24mm and 25mm – source: fotosaurier

Over the decades of my own practical use of SLR-lenses (of nearly all makers-brands!) has lead me to an understanding of the quality for normal photographic use.

This collection of test candidates does NOT claim to be a COMPLETE collection of all design legends of 24mm/25mm. There is a large gap in time with prime-lenses between 1984 and 2015. That means: the legendary first historical aspherical lenses in this range are missing in the comparison. If I ever will be able to get hold of them for a test, I would update this article. The modern lenses tested for comparison are (of course) all aspherical types!

In spite of the fact, that important legendary lenses of the 1980s and 90s are missing here, this report allows to draw some interesting conclusions about important steps in optical lens-engineering, which finally lead to Ultra-Wideangel-Lenses which have uniform resolution and contrast over the complete field of view (FoV).

I have always looked for a method to show the quantitative progress in optical quality of photographic lenses over the nearly last 100 years – and I think I have found a good way to understand this progress with my new comparison-charts (Fig. 4 and Fig. 5 see below). What was surprising: the progress over time is independent of the lens-maker and brand. It is generated by a sequence of milestone-like innovations by singular design-legends, innovative calculation progress, creation of new glass-formulations and finally the lens-making-process – espacially allowing for the production of aspherical lens-surfaces! Once the innovation-step is basically made, it is spreading around the globe very quickly (typically within one or two years!).

There are few lenses, which stand out of the general quality-development curve, reaching a higher level of resolution earlier than most others – to be seen here mostly in Fig. 5:

ATTENTION: These measurements are made with USED lenses today, some of which are more than 60 years old! There are influences from ageing and wear (even abuse …) which have become part of the lens-properties when we measure them after long time. However, I only make measurements with samples of lenses, if the optics are clear and undamaged and the mechanics do not show excessive wear or abuse.

Vier_24+25er
Fig. 2: Starting with big-big negative front-meniscus-lenses (at left Angenieux Retrofocus 24mm f/3.5 and Zeiss Jena Flektogon 25mm f/4) the lens-designers soon learnt to reduce the front-lens diameter (at right: Distagon 25mm f/2.8 for Contarex and Olympus OM 24mm f/2,0), creating better results and generating lens-bodies, which were more acceptable  – source: fotosaurier

2 – Data section for 15 historical 24/25mm-prime lenses, 3 modern 23/25mm prime lenses and 4 modern zooms at 24mm-setting:

Auflösung ETC 23-25mm korr

Out of this Chart I have filtered two separate charts, showing the development of RESOLUTION over the decades.

Fig. 4 shows the center-resolution open aperture (blue) and stopped down to the aperture with the highest resolution (green) in the center:

23-25mm_Resol_Center_korr

23-25mm_Diagram_Center_korr

The second chart is showing the corner-resolution at open aperture (blue) vs. the best resolution-value stopped down (green) in the corners (mean value over all four corners) – where „corner“ means a value of 88% – 92% of the full picture circle of the lens which is 21.5 mm radius:

23-25mm Resol_Corners_korr

23-25mm_Diagramm_Corners_korr
Fig. 5: Corner Resolution-values  of 21 Lenses at FL 23-25mm at open aperture (blue) and optimum aperture (green, which means: the aperture at which the weighted mean of all the 46 measurement-places over the 24x36mm-frame is maximum. (The maximum corner resulution-value of the individual lens may be higher.) – source: fotosaurier

You see, that nearly all of the difference in resolution of historical top-notch wideangle-lenses for SLR is in the corners of the picture (and of course also continuously in-between center to corner areas). This is easy to understand, because the difficulties for lens-correction rise dramatically with the FoV, which is here 84 degrees corner to corner diagonally.

Besides the resolution, there are other important properties, which improved dramatically over these six decades of lens-engineering history:

a – Chromatic aberration (CA in pixel): It is very low in all these lenses in the center. It typically ranged between 4 and 8 pixels in the corners for the very first lenses of this type. It stayed around 2-3 over the time before aspherical lens-surfaces could practically erase it. Today with the best modern lenses, the value is close to zero (under 0.5) without camera correction and zero with correction.

Among the early lenses the Zeiss Distagon 25mm f/2.8 (though not really outstanding in resolution compared to the other early lenses) pops out, because it had already values of 2-2.5 pixel in the corners – together with the „unicorn“ Topcor 2,5cm f/3.5.

Please consider, that the CA-value in pixel for the same lens is the higher the smaller the pixel size of the sensor is  – here 1 pixel is 3.77 µm.

b – Linear distortion (%): distortion shows – from the beginning – the biggest differences between the legendary lenses of the different designers and brands. The designer has to do a compromise-job in each lens, balancing out the design between resolution, chromatic aberrations and distortions. 0,5 pixel is a very good CA-value even acceptable for acrchitectural work (though „zero“ would be better, of course), 0,75-1,0 pixel is a good compromise-value and 1.5 pixel just acceptable for alround use.

Looking at the spread-sheet Fig. 3, it is surprising, that Angénieux with the very first retrofocus-lens of this wide angle decided to go for nearly „ZERO“ distortion in his design! He had gone close to zero in the 35mm and 28mm-designs before that, too! Probably he wanted to give a statement of his art, because this was really difficult at that time … At the same time he accepted a somewhat higher CA of 7-8 pixels (corresponding to 0.03-0.04 mm). In my collection of top-notch lenses such a low distortion does not appear again before the modern Zeiss Batis Distagon 25mm f/2.0 – and only the legendary 1971 Minolta MD 24mm f/2.8 (including the VFC-Version) came very close with ca. 0.18-0.29% distortion in my measurements.

c – The close-focusing system: there are further innovations to consider, e.g. the lens-design for close focusing. Here one of the important innovations is the floating-element close focusing system – introduced 1971 by Nikon and Minolta first for wideangle lenses as far as I know. This is one of the early merits of the two 1971/75 24mm-Minolta-lenses.

3 – Conclusions:

3.1 Center-resolution:

Since the early days of geometrical optic lens-design with Petzval, Abbe and Seidel, lenses could be designed absolutely perfect for nearly unlimited image-quality (resolution and CA) „on-axis“, which means: in the center of the picture-field … And the  famous designers did it all the time – as soon as they used 4 or more elements in a photographic lens-system.

The first time, I found a proof for that, was with my resolution-measurements on Bertele’s first Ernostar 100mm f/2.0 from 1923 (a four-element-design WITHOUT COATING!). Compared to the legendary Leitz Apo-Macro-Elmarit 100mm f/2.8 from 1987, this lens achieved 98% of the resolution in the center – but only in the center! See my Ernostar-Bog-Article here. (This was the very first report in my photo-blog …)

So, it is not really surprising, what Fig. 4 is telling us: all top-notch lenses show a very high resolution level in the image center since the invention of the retrofocus wideangle design in the 1950s – and they are all on the about same level – though being historical lenses with up to 65 years of age on their back! The reason for that result is, of couse, that only legendary lenses of all brands are taken into the comparison! Maybe the Takumar-lens happens to be one of the weaker examples …

The Olympus OM 24mm f/3.5 „shift“ drops down somewhat against its neighbours. That is no quality issue: this lens has an image-circle diameter of 57mm for up to 10 mm shift! It came out 1984 long before Canon brought out its famous tilt-shift-lenses … Look at the corner-resolution result of this lens in Fig. 5 – it resolves extremely even over its FoV!

in this graph I marked two horizontal lines: one for the resolution of 2.000 LP/PH (linepairs per picture height), corresponding to the resolution of a 24 MP-sensor, which today is the de-facto-standard for  modern digicams. It normally has 4.000 by 6.000  pixels – and 4.000 pixels in the picture height, corresponding to 2.000 Linepairs. At the same time it is just (+15%) above the 21 MP which I estimate for the resolution of modern analogue (general purpose) film emulsions.

The other (upper) horizontal line marks the 3.184 LP/PH Nyquist-frequency of the Sensor in the Sony A7R4-digicam. This is physically the limiting resolution-value for the camera itself. Today, however, the software-algorithms in the camaras can generate structures in the picture, which are typically 15 – 20% higher in resolution, compared to the Nyquist-frequency. And they do this without creating an artificially looking „oversharpened“ picture! Good job!

This means:

All the legendary historical 24/25mm-retrofocus-lenses for SLR-cameras do out-resolve the modern 24 MP-Digicams in the center – mostly even with open aperture! And many of these lenses even come very close to (or exceed) the Nyquist-Frequency of my 60,2 MP digital camera.

Among the historical lenses two examples peek out a little bit (they peek out much more in the graph for the corner-resolution!):

The legendary 1965 Topcor 2,5cm f/3.5 exceeds the Nyquist-frequency of 3.184 LP/PH – and stopped down to f11 it is in the center the highest resolving of my 24/25mm-lenses until today. Together with the tremendous result of its corner-resolution it is one of the exceptional lenses, which I call my „UNICORNS„. Until today, I have not found any explanation for the astonishing early level of performance of this lens – how could that have been achieved? (15 years before the next-best Olympus-lens!) – and who did it? – and where did this person go afterwards, when Topcons innovative power faded out, to bring in her/his inginuity? (… to Olympus?). (This observation refers to other early Topcor-lenses al well!)

The other unicorn peeking out here is the Olympus OM 24 mm f/2.0 of 1973. In my lens-collection it is exceeded only by the 40 years younger Zeiss Batis 25mm f/2.0. But, to be honest, the difference is not really that dramatical – considering the four decades …

Referring to the zoom-lenses (set at FL 24mm) in this test: I just was curious, where the modern zooms would stand in such a comparison. We learn that the 1kg-Monster-Tokina 24-70mm zoom at 24mm has one of the best results – even at f/2.8 … in the center of the picture.

At the end of the line-up of 21 lenses I put the Fujinon-Zoom 32-64mm f/4 at 32 mm on the Fujifilm GFX100 (33x44mm – 102 MP), which corresponds to FL 26mm on „full-frame 35mm“. This shows, that for an essentially higher resolution in the picture-center, we today have to go to a larger sensor-format.

3.2 Corner-resolution:

Fig. 5 contains the important informations of this comparison-test. It shows, that step by step all the improvements in innovative design, glass-formulations and aspherical surface-generation were needed to bring finally the corner-resolution of the picture up on par with the center resolution at 24mm focal length, which is possible today – but only with the use of aspherical lens-elements!

In the graph for the corner-resolution I have added a third horizontal line, which marks the resolution at 50 Lines/mm – corresponding to 600 LP/PH. This is needed to judge the corner-resolution of the early historical lenses.

In the 1960s a wideangle-lens was rated „very good“, when it achieved a resolution of 40 Lines/mm (Modern Photography and others). I have written an article about this already here (in German).  Open aperture most super-wideangle-lense started open aperture in the range of 26 to 32 L/mm in the 1950s and 60s. Stopped down practically all the tested historical lenses surpassed the 40 L/mm-limit.

From 1958 on (ENNA) the stop-down corner-resolution rises continualy (with the exception of the two „unicorns“, already identified in Fig.4) until end of the 1970s,  it arrives close to the 2.000 LP/PH-level, which means: from now on the top-notch-lenses out-perform standard analogue fine-grain film (1977 Nikkor and 1984 Olympus). This last step was then achieved by the use of extraordinary dispersion glass-types.

The two „unicorns“ in this test arrive much earlier at this level: the Topcor 2,5cm f/3.5 out-performs analogue film already in 1959 and the 1973 Olympus OM 24mm f/2.0 exceeds this and comes close to todays modern aspherical lenses.

The modern aspherical prime-lenses are represented in my test by two very different samples:

There is the 23mm f/4 Fujinon, which originally is a GFX-lens – but in this test it is measured in the 24x36mm-Mode also with 60.2 MP on the GFX100, achieving the state of the art for 24x36mm lenses (Batis and Sigma-i) as a middle-format lens!

Just as I made my measurements for this test, the SIGMA i-Series 24mm f/3.5 arrived as a representative of a new thinking: no „impressive“ technical data   – but (hopefully) impressive preformance instead. The result shows: it achieves reference status on a 60.2 MP-sensor with corner-resolution at 85-95% of center-resolution, plus zero-distortion, zero-CA and very close focussing!

Also great news: modern zooms like the Sigma G 12-24mm f/4 – measured at 24mm – arrive now at this level of prime-lenses also in the corners!

As I had no samples of the early historical aspherical lenses in this test, we can not see, in which steps the aspherical lens surfaces moved the wideangle-performance in the picture-corners to the present level.

Maybe this gap can be filled out in some future times.

NOTE 1 – All resolution-values, which are published in this article, refer to MTF30 – what means: the point on the MTF-curve (see Fig. 7), which hits the 30% contrast value.

NOTE 2 – in Part II of this Article I will share some more informations about each individual lens (including pictures, MTF-curves and  lens-schemes).

Appendix: Method of measurement and definition of results

I use the set-up and software by IMATEST with the original IMATEST-Target. I use the large SFRplus-Setup-Image with a physical hight of 783mm bar-to-bar vertically. The distance from target to lens-flange is 0,97 meters. In this area 46 targets are analysed and I share MFT30-weighted-mean-resolution-values (all-over, center and corner), edge-sharpness, linear distortion and maximum lateral CA-values.

Resolution-values are given in Line-Pairs per Picture Height (LP/PH) – where the picture-height is always 24mm. Edge-sharpness is given in pixels (width 3,77 µm).

#TestChart_Angén90f2,5_f2,5
Fig. 6: IMATEST test-target 783mm-bar-to-bar distance. Resolution is NOT measured in the small concentric targets, but at the outside-edges of the black boxes, which are tilted b ca. 5 degrees – source: fotosaurier.

For the measurement I used a SONY A7Rm4 with 60,2 MP-resolution which has a pixel-width of 3,77 µm. The theoretical resolution-limit of the sensor is 3.184 LP/PH (Nyquist Frequency).

The camera setting is used basic as delivered from factory at ISO100 and exposure-compensation of -0.7 stops, using out-of-camera JPEGs. All measurements are made with the identical camera-body (which is important for a precise comparison: I have used one other (earlier) body of this model in comparison, which gave resolution-values between 50 and 200 LP/PH lower than my own camera-body). The repeatability with this method I estimate at 2-2.5%, using ALWAYS manual focusing on the lens with maximum focusing enlargement (11.9-fold) in the camera-viewing-system. Measurement is repeated with re-focusing until a stable maximum resolution at open-aperture of the lens is found and then pictures of the resolution-target are taken with the focussing made wide open for all full down-stops of each lens.

Edge profile (edge-sharpness) is the width of the rise from 10% to 90% intensity at a dark-bright edge in the test target – measured in pixel (width 3,77 with the camera used) – Example shown here for the latest 24mm-prime-lens SIGMA i-Series 24mm f/3,5 – at open aperture f/3,5:

Edge+MFT_Sigma24f3,5
Fig. 7: Edge-profile (top) and MTF-curve (bottom) from the IMATEST software – here the perfect graphs for the brand new Sigma 24mm f/3.5 – at open aperture. I will publish these Curves for all the lenses in PART II of this article – source: fotosaurier

Cromatic Aberration (lateral in the picture-plane) is also measured in pixel separate for red against green and blue against green over the full picture field – in the spread-sheet I note the maximum value, which is in most cases for blue and for most historical lenses in the corners of the picture – sometimes however in the intermediate area.

For more details of testing read my special blog-Article here.

Copyright: Herbert Börger

Berlin, March/April 2021

Long Telephoto-Lenses and Temperature

Would you expect, that the optical performance of your photographic lenses can be seriously influenced by the operating temperature? Have you ever realized lack of sharpness in extreme environmental temperature conditions?

The simple answer is, of course, that within the specifications for use, given by the makers, there should be no such concern. But it is not that simple.

For amateur astronomers with their mostly very long telescope-focal-length optics (mirror or lens) this fact is very common:

before using the instrument in the clear and mostly cold winter-nights, you have to put the telescope early enough outside (shielded against due) to bring it into a thermal equilibrium with the ambient air at the time you start your observations. The reason: during essential temperature-changes of the optical components (mirrors, lenses) and their mounting devices, their surface-shapes and adjustment change and destroy the extremly precise optical alignment – until the thermal equilibrium is restored. The refractor-lenses may be mounted to allow for some thermal differences, but large mirrors have to be mounted and adjusted extremely precise, so that the cooling-down of the mount, that holds the mirror, may even generate mechanical tension on the mirror – and that generates optical distortions! So we should remind: the absolute temperatures are not the problem – but the thermal transition stages from warm to cold or opposite way!

This fact is also an important design aspect for telescopes: the preferred structure is „as open as possible“ to allow the air to circulate and to generate a good heat-exchange with the internal telescope structure to speed up this process. While the air gets colder during the night, the instrument’s optics can follow close enough to keep the temperature difference low.

There is an impressive document in the archives of the Mt. Wilson Observatory (near L.A., USA) describing the „first-light“-moment of the new 2,5 meter mirror telescope (Hooker-Telescope) on November 1, 1917 – use this link to the adventurous story! („First light“ is the moment, when somebody looks through the finished instrument for the first time.) Here the first-light moment at Mt. Wilson is described near the end of the long text in this link and shows, what a three hour cool-down time made to the optical properties of the 2.5 meter mirror, (which was made by George Willis Ritchey – and allowed for the detection of the expansion of the Universe by Edwin Hubble shortly after taking this telescope into service.).

Picture 1: 2,5 m (100 inch) Hooker-telescope on Mt. Wilson: just struts hold the mirrors to ease the circulation of air for for a fast achievement of  temperature equilibrium – source: Ken Spencer, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Many instruments in astronomy are closed assemblies, using a corrector-plate (Schmidt-system) or meniscus-lens (Maksutov-System) in the entry of the tube and the mirror at the rear-end (catadioptric telescope – see also my specific blog-article here.) The big disadvantage of these closed systems is the „inertia“ in cooling down due to the closed volume in the telescope tube. Therefore often slits around correctors and mirrors are placed, which allow for sufficient circulation of air through the tube – and even active ventilation is used to shorten the period to reach equilibrium. In some big modern telescopes, the mirror may even be actively temperature-controlled.

Picture 2: „Closed“-tube optical system Maksutov-Cassegrain-Teleskop – source: Wikipedia – Author: Halfblue – http://creativecommons.org/licenses/by-sa/3.0/.

Long telephoto-lenses for normal photography can not be open systems, because the lens-barrels definitely have to be tightly sealed to avoid the invasion of dust, humidity or corrosive gases.

This means, that you have to plan and prepare carefully to bring your equipment to ambient temperatueres in time to avoid these thermal problems. For photographic equipment this would equally refer to the situation, when you come from climate-controlled environment (e.g. hotels) into wery hot (and humid) areas. There is an additional problem, that in bringing cold equipment into hot-humid environment, there might be condensation of humidity on the lenses/mirrors.

This problem is even more delicate with catadioptric lenses (mirror/lens-systems often called just „mirror-lenses“ – in German „Spiegel-Objektive“). In these the surface-shape of the mirrors and the adjustment from mirror to mirror is extremely sensitive for the optical performance of the lens-systems.

I have to-date not realized this with focal lengths of up to 350 mm (though it might be also there to a certain dergree) – but this is definitely an important aspect for focal lengths between 500 mm and 1,000 mm or longer.

From which focal length on these problems may occur, will mainly depend of the type of optical system  – and of course the resolution of your cameras sensor!

Here I want to show you this effect with an example of a catadioptric lens of 800 mm focal length: the Vivitar Series 1 Solid Catadioptric 800mm f/11, used on the Sony A7Rm4 (60,3 MP, 35mm format – 3.77 µm pixel-pitch).

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Picture 3: Vivitar Series 1 Solid Catadioptric 800mm f/11 – source: fotosaurier

It was the first day this year with just sligtly above zero outside temperature (+2 degree Celsius) and very clear air. At ca. 1:15 p.m.I set out the 800mm f/11 lens on the tripod on the balcony and tried to focus on my favorite landscape test target: a roof-top at about 40 m distance.

The advantage of this target is, that it has large AND fine details, low contrast AND high contrast areas and – most important – a sufficient depth, so that I can detect focusing errors very well!

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Picture 4: Overview picture – complete field of view of the „roof-top“ landscape target in ca. 40 m distance taken with Sony A7Rm4 and Vivitar Series 1 Solid Cat 800mm f/11 – this is the „sharp“ picture after the cool-down period of the lens – source: fotosaurier

It was nearly impossible to meet the positive focus position – so I did the best guess and made the photo – and here is the 100%-crop around the focus-position, which is the first steel spring at the right side of the roof edge:

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Picture 5: The 67% detail of the focus-area (clamp and spiral-spring!) made 15 minutes after setting the lens outside. Best guess of focus, however, you will find no sharper point in front or behind – the distance scale on the lens says 50 meters in this non-equilibrium temperature situation – source: fotosaurier

At this point of time the lens internally is still on room temperature of about 21 degrees … starting to cool down for about 15 minutes, which it took me to set everything up and focus carefully – but desperately, becaus no really sharp focus was seen in high viewing-magnification.

I had focused using the maximum viewfinder enlagement in the Sony camera and was sure: this is not a really sharp picture. But I could not find a better focus. Picture 5 is a 67% crop of the image taken. And as the subject has some depth: no – there is no better focus to be seen on this picture in front or behind the plane of the spring.

I left the lens with camera in this position for three hours and refocused the lens: now I experienced a quite snappy focus – and you can see the same crop-area here:

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Picture 6: The 67% detail of the focus-area (refocused!) after additional 3 hours of the lens outside – source: fotosaurier

The gain in sharpness is damatical – and it exists over the whole field of view, not only in the plane of focus! Also out-of-focus areas show higher contrast now.

However, it connot be ignored, that this catadioptric lens in this picture does by far not use the potential 3,168 Line-Pairs per Picture Height Nyquist frequency of the cameras sensor. My estimate is, that we have here an MTF30 of about 1,100-1,200 LP/PH. So either the three hours of cool-down time were not yet sufficient – or the lens may be not better than this.

(The 1,200 LP/PH MTF30-resolution would correspond to 100 Lines/mm in older „analog“ data. Very good CATs in the 1970s had center-resolutions (measured on film) between 50 and 60 Lines/mm. This relation makes sense, as the difference (factor 0.6 lower for film!) may be owed to the effect of grain and the thickness of the emulsion.)

The „Solid Cat“ 800mm f/11 is a massiv piece of optics – the lens barrel is nearly completely filled with glass, as you see in the lens-scheme:

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Picture 7Lens-scheme of the Vivitar Series1 Solid Cat  – source: Perkin Elmer Patent application

It is an absolutly unusual mass of glass – so I would not exclude, that the cooling time should even be longer to reach the thermal equilibrium. My plan is, to make a sequence of photos taken in shorter intervals and over a longer time – as soon as the outside temperatures go down again.

I am not so happy with the fact, that I had to use landscape-scene-shots to demonstrate the performance of the lens, however, for 800mm focal length my IMATEST testing-arena is too short. Maybe I will make a parallel IMATEST-trial then with a 500mm CAT.

So, please, consider this as a first teaser for the topic which has shown clearly, that photographic lens performance may seriously suffer during the time, a lens is undergoing strong temperature-change and before equilibrium is reached.

I promise to come back with a more elaborate research-plan soon.

Herbert Börger

Berlin, December 4th, 2020

Aphorism of the day: Scientific research is most successfull, when it brings up more new questions than it has answered. (fotosaurier)

Copyright: fotosaurier