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| Resolving power of photographic films |
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Decades ago, when black & withe films were the dominant photographic recording materials, the resolving power was seen by many photo enthusiasts as the deciding factor of photographic quality. Although we at Carl Zeiss are very much aware that resolving power is only one of many criteria which constitute image quality, it is quite important when photos are taken for documentary purposes, where resolving power primarily determines image quality. In documentation, resolving power is one of the most important aspects of image quality. Since Carl Zeiss, by and large, is a company with strong scientific roots and many science related activities, documentation is at the hearts of our photographic mentality. We therefore take the measures needed to design and manufacture our lenses with above average documentation qualities, which includes high resolving power.
This, of course, makes sense only when the resolving potential can actually be utilized with existing films, and is not just mere theory. At Carl Zeiss, we therefore conduct occasional tests to find out about the real-world resolving power of photographic films. As announced in CLN 17, we want to share our results with CLN readers. The tests were conducted by Kornelius Mueller of Carl Zeiss for the the purpose of lens resolution testing and information recording. Error margin is +/- 10 linepairs per millimeter.
As we add new test results about new film types in future CLN issues, we will also publish more detailed information on the test target used, and the evaluation of the resolving power, the tools and methods.
The film manufacturers generally conduct their tests under special, sterile, precisely repeatable laboratory conditions where high performance microscope lenses with resolving powers beyond 1.000 linepairs per millimeter and light sources with very controlled spectrum are used to image the test targets onto film, completely unrelated to any real world photo situation. In contrast, our procedure is based on typical photo conditions like outside sunlight, exposures controlled by normal camera shutters, focusing done with the normal focusing aids of the camera, standard film developing by a normal photo finisher, and of course, using normal Carl Zeiss photographic camera lenses. In other words: we use equipment and techniques which are readily accessible and our results are therefore relevant to every photographer.
The only non-standard tool is used for viewing the films at high magnification: Since we are after resolutions of 200 linepairs per millimeter and significantly beyond, we need a suitable viewing aid for this purpose. With the normal human eye being able to resolve around 5 linepairs per millimeter at normal viewing distance, we need at least 50 x magnification to be able to see 250 linepairs per millimeter dissolved. No loupe would do.
So we use a Carl Zeiss SV 11 Apo stereo, which delivers magnifications up to 100 x. Admittedly, this tool may not be part of the normal equipment of a photographer, but it is important to note that the special equipment involves the interpretation, not the performance, of the tests. Based on our extensive experience over several decades of testing, here is our list of film maximum performances, presented in tabular form:
Table with the performance of photographic films
| Comments of Carl Zeiss about the film market, concerning resolving power: |
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In recent years, high resolution low speed films have been discontined by all large film manufacturers. We found two reasons for this move:
1. Film manufacturers live under the impression, that only few if any lenses are capable of utilizing film resolving powers of 200 linepairs per millimeter and beyond. When Carl Zeiss displayed ”The sharpest color photos ever taken” (taken on Kodak Ektar 25) at photokina 1996, those few employees from film manufacturers who saw this display were deeply impressed and finally saw the fruit of their work, resolving power in films, utilized in real world photos.Carl Zeiss may want to repeat such a demonstration and bring it to the awareness of broader circles of the photographic world.
2. Film manufacturers find that low speed films around 25 ISO have become increasingly unpopular. They discontinue these films and concentrate on 100 ISO and faster follower products instead, incresing their resolving power.
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