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):
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:
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:
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:
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:
These are the curves, which are generated from each digital picture (black&white):
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:
- 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:
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:
Following picture is the MTF-curve of the analog image „as scanned“ (in the center of frame):
The „noise“ in the curve is caused by the film-grain, which is about the same size as pixels.
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:
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.
Copyright „fotosaurier“
Herbert Börger, Berlin, November 2023