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 method: 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 first 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):
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 ist max. resolution of 5.000 ppi – that corresponds to 2.383 LP/PH as a Nyquist Frequency and delivers ca. 33 MegaPixel pictures on FullFrame.
There is no affordable filmscanner with higher resolution on the market. But: the RPS 10M offers a multi-scan mode, which extends the achievable resolution considerably!
It was possible to set the „development-parameters“ of the scan-software (Silverfast) like sharpening and contrast so that the maximum resolution in the film-based pictures is about the same as that of the purely-digital data-processing chain – very close to the Nyquist Frequency of the Sony-sensor. More about that later!
What the resolution-graphs here clearly show: the very low resolutions in the corners (and also part-way!) of the digital sensor (especially open aperture!) are an artefact of the sensor! With this picture we know, that this happens not only with rangefinder-lenses – but also with SLR-lenses! What a wonderful uniform resolution-result with the lens on analog film! The strong drop-off in resolution at f/16 and f/22 on the digital sensor also shows to be a sensor- artefact and not a diffraction limitation.
I confirm again: it is the identical lens in both cases! And these results are pretty much representative for many other analog lenses!
1. The digital IMATEST lens testing method and software for digital imaging – extended to pictures taken on analog film:
A. Measuring the optical performance on a digital sensor is facing several facts and influences, which the design of the lensed not consider: additional optical elements in the optical path like filter stacks and micro-lenses and the effects of aliasing!
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 this regarding historical SLR-lenses, which were never planned and calculated for the use with modern digital sensors. For rangefinder-lenses the degradation of the picture quality in the corners of wideangle-lenses is so dramatical, that it is clearly seen, while 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 62,5 Mega-Pixels, resulting in 61 MP effectively on Full Format (35mm stills).
Until now I did not know, whether the measurement of my historical SLR-lenses 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 standard 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 (60 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-frame.
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, iso100, developed in Rodinal 1+25 (8′)
- Scanner: reflecta RPS 10M film scanner, set at 5.000 psi using multiple scan
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 hoped 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. 5), which is digitally measured on A7R4 with ca. 3,000 LP/PH or higher) and corner-resolutions of 400-600 LP/PH on the sensor .
The question is: are the low values on edges and in corners of the frame, measured with the digital sensors, an artefact, caused by the differences along the light-path?
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. (For this also look at my article on the rangefinder-lens VM Ultron 35mm f/1.7 …)
The 35mm-negative-film:
I made successful 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.
I did the devellopment of the b&w-film myself with Rodinal 1+25.
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).
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).
The picture height, which the scanner digitally delivers (24mm), was 4,766 pixels and so the „Nyquist Frequency“ of the scanner set-up corresponds to 2.383 LP/PH – corresponding to an effective sensor-size of ca. 33 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) – compare this to the MTF-curve in Fig. 5, bottom left:
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 set-up could 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 far above the „Nyquist Frequency“, which would be 2.383 LP/PH, corresponding to a Picture size of 33,7 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.
Dies entspricht etwa 260 Linien/mm Auflösung.
This was in fact possible with the following settings:
Fig. 8: Scan-settings in Silverfast for APX100
Prior to the scan I set the white point and the black point for each pre-scan.
I tested these settings with real-world-shots and found no over-sharpening effects and a good tonal balance.
The following picture shows the MTF-measurement results for the OM 28mm f/2.8:
Fig. 9: MTF-Measurement-results for OM 28mm f/2.8 on APX100 b&w-film – source: fotosaurier
We see here the proof for the artefacts, which we have seen with the readings on the Sony A7R4-sensor: dramatic resolution-loss not only in the corners but in this case also over large „part-way“ image area between center and corner below aperture f/8.
I will continue to publish examples for such measurements on analog film for rangefinder and SLR-lenses very soon, as I did it already in the case go the Voigtländer VM Ultron 35mm f/1.7.
Above focal lengths of 65 mm onwards, this effect on digital sensors will vanish.
What these results also show: using a film-scanner gives you, of course, a digital image (based on the scan-parameters) of 33,7 Megapixels. But your analog black-and-white negatives will be much sharper, than you will get from any digicam with 33 MP!
One final hint: the use of a PCX-lens in front of this OM 28mm f/2.8 lens on a Sony-sensor does not give any advantages – in contrary: the image is degraded in resolution mostly:
Copyright „fotosaurier“
Herbert Börger, Berlin, November 2023