Resolution on Analog Film

Analog vs Digital – Angénieux M1 50mm f/0.95

This is one of the most iconic historical lenses (post-WWII), which I know.

I have worked about it in the last two years, because it became available to me physically through foto-friend Thomas.

The facts:

First time I mentioned the „M1“ in my basic essay on Pierre Angénieux here . To date this is only available in German – you will find all the historical facts there:

The 8-lens-Doublegauss was designed since 1953 (patent application) for cine in the different cine-formats in the focal lengths 10mm, 12,5mm, 25mm and finally 50mm and was the first mass-produced lens with aperture <1.

Thomas‘ lens is a unikat item: the historical lens-barrel with aperture ring is integrated into a focussing unit with M58 threaded interface, which allows the adaption to Sony E-mount.

**Fig. 1**: Angénieux M1 50mm f/0.95 in focusing-unit – ultra-slim E-Mount-adapter to M58 on the left – source: fotosaurier

All my findings on the optical performance of this lens measured with Sony A7R4 full-format 62 MP-Sensor you will find here . The lens covers 37mm picture-circle, which is falling a bit short for the need of full-format (43mm) – always remembering, that it is designed for cine „Super35“-format (18.7mm x 24.89mm corresponding to picture-circle 31.2mm).

**Fig. 2**: Angénieux M1 50mm f/0.95 at f/0.95 on Sony FF-Sensor of A7R4 – source: fotosaurier

In addition to the full-format measurements, you will find in the link also the results for the Super35-cine-format, taken with the crop-mode at Sony A7R4 (which has 25 MP resolution in this mode), which is the format it is designed for.

As we know in the meantime, that the resolution of historical lenses in the corners (and also the main picture-area) of the high-resolution sensors may be degraded, I looked for a way to analyse the analog picture on film with the IMATEST-software.

I succeeded with this after nearly two years work – HERE I described the way to do this: taking the pictures of the IMATEST-target on analog film (B&W Agfa APX100) – developing under strictly standardized conditions (Rodinal 1+25, 8′) and generation the digital picture with a film-scanner at 5.000 ppi (reflecta RPS 10M) using multi-scan mode and again strictly standardized sharpening-parameters with SilverFast. This results into ca. 32 MP-pictures at 24mm x 36mm (FF).

The link shows the procedure with a 28mm wideangle Olympus-OM-lens 28mm f/2.8 and unveils essential differences in picture quality in most areas open aperture (with exception of the inner center of the picture).

Now I had to solve the problem, to get the Angénieux M1 50mm-lens focused on film. There is definitely no way, to get the lens via adapter on a 24mmx36mm analog camera. Look at the rear side of the lens in the focusing-unit:

**Fig. 3**: Rear view of the Anénieux M1-50mm f/0.95-lens in the focussing-unit with rear M58mm-thread. The rear lens stands at the level of the M58-thread! – source: fotosaurier

2. Resolution-results on analog film.

What ever you would have constructed, to get this lens adapted onto an M39 or LM-camera body, it would not focus to infinity and not even to the 1,8m-distance, which would be necessary to focus on my IMATEST-target. (I did not take in consideration to use an analog-cine-camera for this purpose, which would of course be fitting to this lens, made for cine35-format … but would have required the use of extensive additional film-equipment and processing systems, which I were not acquainted with!)

I finally did it – read chapter 3 further down, if you are interested how!

Here is the lens transformed into a unikat-combination of Canon7-body and the 50mm f/0.95-lens in focussing-unit. It just allows to focus around the point of 1.84 meters distance to get the IMATEST target sharp:

**Fig. 4**: Angénieux M1 50mm f/0.95 lens adapted on a „stripped-down“ Canon7-body, allowing to focus at 1,8m at farthest … or coser – source: fotosaurier

As there are no focussing aids available (and not even an apropriate finder!) from the side of the camera, the lens has to be focussed, using a ground glass in the waist level finder placed onto the film-plane (with open back!) before the film is loaded. (The good-old Exacta WLF worked excellent for this purpose!)

**Fig. 5**: Focussing the lens on the matte-screen of WLF (Exaktawaist level finder) at open camera befor loading the film – source: fotosaurier

Also the exact framing and adjustment of the film-plane parallel to the target had to be done through the waist-level finder at the open back of the camera.

The adjustment of the film-plane parallel to the camera is one of the most important aspects for precise measurements – especially for the fastest lenses. The depth of focus is extremely narrow at f/0,95 so that sharpness may be given in the focussing location but the film-plane runs out of focus, if it is not precisely parallel to the target.

Using a digicam it is easy to adjust the camera: you take a photo of the target, take this picture out of the camera and analyze it in IMATEST. The software measures the angle between horizontal and vertical lines, which are per definition parallel in the target. That means, the angle should be ZERO, if the camera is perfectly alligned, i.e. the lines are also perfectly PARALLEL on the film-plane.

For a good allignment of the film-plane, I use 0.15° as a limit for the angles between paralell lines in the pictures on sensor or film. In this measurement-session with the Angenieux M1 50mm f/0.95 the values, which I achieved were

0.148° for horizontal lines – 0.004° (= perfect!) for vertical lines.

That used to be sufficient normally. I have, however, to date little experience with apertures of f/0.95 … The horizontal mismatch is within my tolerance, but we will see in the detailed analysis, that there is a important effect in resolution over the picture.

After loading the film, you have to handle the whole set-up like a raw egg, always being aware, that the final digitally converted picture will resolve at 5,08 microns (pixel-size) … and there is a lot of manipulation necessary before each shot:

setting the aperture,
advancing the film,
pre-tensioning the self-timer
pushing the release knob

… and praying, not to stumble over the tripod-legs all the time!

This is the full-frame picture on film:

**Fig. 6**: Full-Frame-Picture Angénieux M1 50mm f/0.95 on Film at f/0.95. Compare with Fig. 2 (on sensor) ! – source: fotosaurier

Even on this small picture at f/0.95, you see already, that the details very close to the edges and corner look sharper than on the Sony-sensor-picture (Fig. 2). Only the very central details show higher contrast on the sensor.

As the lens is designed for the cine35-format it doesn’t make sense to analyze the FullFrame film negative. So I cropped the frame to the width of the Super35 film frame-size in my scanner-preview.

In the following picture you see this situation of the scanned part of the picture. The black rectangle inside the area, which is scanned, shows the size of the super35-frame (24.89mm x 18.70mm) which corresponds to a picture-circle of 31.1mm.

**Fig. 7**: Scanned section of the B&B-negative – the black rectangle shows the size of the super35-film-frame with picture-circle 31.1mm. The whole scanned frame, which is analysed has 34.2mm picture-circle – source: fotosasurier

To enable the automatic resolution-analysis by IMATEST, it is necessary to have the black bars top and bottom of the target. Due to that, the picture area for resolution measurement has a picture circle diameter of 34.2 mm, exceeding the super35-frame by 3.1mm (corner-to-corner).

The following picture shows the set-up for the resolution analysis in IMATESTS. The magenta coloured rectangles mark the contrast-edges at which the resolution is measured – in 44 places all over the picture.

**Fig. 8**: IMATEST analysis set-up for resolution at 44 locations (calles „ROI“), four of which are located in the center (marked by the inner circle), four are located in the far corner areas at ca. 85% of the picture circle. Most of the measurements (36) are belonging to the largest picture-area called here „part way“ – source: fotosaurier

In the next picture you see the individual resolution-values in all these ROIs at f/0.95:

Fig. 9: IMATESTs 44 individual resolution-values (MTF30) within the 34,2mm-picture-circle – source: fotosaurier

The MTF30-mean-values are 1,237 LP/PH for center, for „part way“ (the majority of the picture-area!) mean MTF30 is 560 LP/PH, in the corners it is 378 LP/PH.

In this picture we clearly see, that the MTF30-values at the left side are definitely lower than on the right side edge. Obviously the effect of the small horizontal tilt of the film plane. However, we have to live with that for now, as this was the best allignment, which I achieved during this occasion to measure this lens.

However, the effects on the average resolution readings of this miss-alignment are smaller than it seems on first sight. Due to some field-curvature of the picture plane (which is not an exact plane) the right-edge values are increase, because they moved closer to the best focus, wheras the left edge moved away. I we had a perfect alignment, the right-edge-values would go down and the left-edge-values would increase: as a result of this, the average-values shown in Fig. 11 are not so far from the truth.

The best illustration of the resolution-distribution over the picture is the graph, which IMATEST calls „radial MTF-plot“: it shows the MTF30-value over the distance from the center (in percent!):

**Fig. 10**: The radial MTF-plot for Angénieux M1 50mm f/0.95 at oper aperture f/0.95 on film Agfa APX100 – MTF30 in Linepairs per Picture Height – 100% distance to the center correspondents to 34.2mm picture circle – source: fotosaurier

The software analyses the vertical and the horizontal edges in the target separately. Horizontal values are „mostly“ sagittal and the vertical ones „mostly“ meridional. For both we have very large variations in this graph. This is partly due to the horizontal missmatch of the film-plane, which we have seen already in Fig. 9.

Now let us have a look on the corresponding radial MTF-plot for the digitally taken picture with Sony A7R4 with super35-frame-mode:

**Fig. 10a**: The radial MTF-plot for Angénieux M1 50mm f/0.95 at open aperture f/0.95 on Sony A7R4-sensor – MTF30 in Linepairs per Picture Height – 100% distance to the center correspondents to 31.1mm picture circle – source: fotosaurier

Scattering of measuring points is smaller due to very good parallel allignment of sensor to target (horizontal 0.04°, vertical 0.05°) – but clearly seen is the much lower level of resulution in part way and corners (attention: different scales on vertical MTF30-scale!).

Now we have all bits an pieces together, to show the perfomance of this lens on analog film (Agfa APX100) and on high-resolution sensor (A7R4) in comparison.

This is the resolution of the Angénieux M1 50mm f/0.95 lens at all apertures (0.95 …. 22 – a remarkable span !!) measured on analog film (B&W Agfa APX100) :

Fig 11: MTF30-resolution of Angénieux M1 50mm f/0.95 for all apertures **on analog film APX100 – 22 MP-scan**. Crop from FF-picture is corresponding to a **picture circle of 34.2mm**. Nyquist Frequency of the analog film scans is 2,296 LP/PH (blue line) – source: fotosaurier

Now we can compare the results of this lens measured on the sensor of the Sony A7R4 in Super35-mode, which is restricted to a picture circle of 31.1mm, whereas the analog values in Fig. 11 come from a picture circle of 34.2mm:

Fig. 12: MTF30-resolution of Angénieux M1 50mm f/0.95 for all apertures in **Super35-film-format on Sony A7R4** (NyqFreq 2,080 LP/PH – blue line) – corresponding to a **picture circle of 31.1mm** – source: fotosaurier

Please consider: the green line represents the resolution in a small central picture-area, the yellow lines only the outmost coners (>80% of the picture radius).

The grey curve („part way“) represents the average resolution value of the biggest area of the picture, so it is responsable for the general sharpness impression of a picture.

Though the corners of the pictures taken on the analog film are 1.5mm more distant to the center of the picture, the average corner values at f/0.95 are more than 100% higher than on sensor – which is also similar for the part way resolution-values.

The 10% higher Nyquist-Frequency of the analog-scans does not explain the 20% to 100% higher resolution readings on the analog film pictures. Obviously the lens is not harmonizing with the sensor (inclusive filter-stack and micro-lenses). To me this clearly is an artefact of the sensor.

With my increasing experience with many historical „fast“ or wideangle lenses I can state today, that this is more or less happening with all historical lenses – not only those for rangefinder-cameras. With RF-lenses it is however much more dramatical. There are a few early historical lenses, which do not show these artefacts on sensor. This type of artefacts seems to vanish more and more with focal lengths above 65mm and openings <f/2.0.

The only values, which are higher with the sensor, are the central resolution readings at f/.95 – f/1.4. I have observed that for all fast lenses to date: the center-resolution open aperture is falling back on film compared to digital sensor. I am pretty sure, that this is due to the thickness of the sensitive film-emulsion, which is mostly pretty much thicker than my scan-resolution of 5.08µ – and at f/0.95 the bundles of light-rays hitting the surface of the film may be larger than the scan-pixels. Light diffusion in the emulsion-layer may also have an influence.

At maximum-stop-down with f/16 or f/22 nearly all historical („analog“) lenses show the effect of a dramatical drop of the resolution on sensor – and it is not the normal diffraction limitation. This latter you see on the graph for the resolution on film, where you see a small (ca. 15%) drop of resolution from f/16 to f/22, which may be just diffraction-caused. In the sensor-based measurements, the drop starts in the center behind f/8 and generally behind f/11 – dropping by up to 50% towards f/22. This is seen with many historical lenses on FF-sensor (at least in center resolution). But there are also exceptions.

I will add my analog/digital measurements comparison with the Canon rangefinder lens 50mm f/0.95 as soon as it will be finished.

3. How the „one-purpose-camera“ was built:

I mentioned initially, that the physical rear end of the Angénieux M1 lens barrel has to be brought closer to the film plane than 28 mm. This allows no space for adaptation on the original thread M39 or LM-bayonet.

I stripped down one of my Canon7-bodys to get as close to the film plane as possible:

Fig. 13: Canon7-Camera-body „stripped down“ – M39-lens-flange and front-screws removed to generate a flat surface, which is the closest possible plane to the film-plane – source: fotosaurier

The M39-lens-flange and front-screws removed to generate a flat surface, which is the closest possible plane to the film-plane. I planned to use the screws as a safety-lock to keep the new adapter in place.

As an adapter-ring I bought a step-ring 77-58:

Fig. 14: The new Adapter-ring in place on the Canon7-body with the cut-away for the lightmeter-window. To the right you see the stripped-off M39-flange – source: fotosaurier

The inner female M58-thread on the step-ring allows to thread down the focussing unit with the M1-lens until it touches the surface of the camera body

The step-ring only interfers with the protruding light-meter window – this forced me to make a cut-away on one side of the ring.

Fig. 15: Close view onto the cut-away sevtion on the 77-58 step-ring. You see the M77-male-thread in the cut section, which is not used here. The adapter-ring here seems to „float“ over the camera front. This is the 0,7mm-gap, which the ultra-strong double-adhesive-tape takes, to glue the large flat backside of the step-ring to the flat front of the camera-body.

The step-ring is finally glued with ultra-strong double-adhesive-tape to the flat front of the camera-body – and I could leave the two safety-scews finally away

You see the new „single-purpose-camera“, which is not at all a „point-and-shoot-thing“, because it has to be focussed and framed with a ground glass placed at the back with open back. It focuses at maximum distance to 1,84 meters, which is just matching to image my IMATEST target – or closer of course.

Fig. 16: Final result … single-purpose-camera for a special lens – just remembers me of my Hasselblad SWC, which I used to own in earlier times …

Happy „fotosaurier“, analog picture taken with Angénieux M1 50mm f/0.95 at f/2.8:

Berlin, March 21, 2024

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!

Herbert Börger, Berlin, November 2023