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Lens testing – which films are in use at Zeiss, and why?
In the course of camera lens application testing, Carl Zeiss strives to gather working experience with most films that are currently important to still photographers. Right now some 40 different film types, mainly medium format roll films 120 and 220, and 135-36 films, but also APS and 4 x 5 including instant films, are in use with Zeiss lens testing activities of all kinds.
One area of particular interest is lens resolution testing. Any film used for this purpose must offer a resolution at the upper end of what can be achieved in general photography today. And the film should offer its resolution without noticeable variation due to film processing. For the latter reason we first searched the market for a film which could be developed in one of the two standard processes, E–6 or C–41, which are available almost everywhere on the globe. As a consequence of this requirement we concentrated on color films. The highest resolving film we found in 1996, when we started our current research, was Kodak Ektar 25 professional color negative film. Kodak claimed that this material resolved 200 linepairs per millimeter (lp/mm) and we achieved this resolution reliably and consistantly with many Carl Zeiss lenses. Sadly, Kodak discontinued this film in 1998.
So we switched to Fujichrome Velvia in 1999 which we found to resolve 160 lp/mm, exactly what Fuji claims.
Also, we examined Agfaortho 25, an orthochromatic black & white film from Agfa, featuring enormous resolving power and virtually no grain. Testing this film, we reached beyond 250 lp/mm using Hasselblad cameras with the lenses Zeiss Distagon T* 3,5/60 and Zeiss Superachromat 5,6/250. However, Agfa has discontinued this film in the meantime.
Also, we examined APX 25, a low speed black & white film, again from Agfa. We reached 200 lp/mm. Agfa claims exactly this figure for APX 25. This film was also recently discontinued, but we managed to secure sufficient stock before it was too late. In general, we found that figures about film resolution, as given by manufacturers Agfa, Fuji, Kodak, are reliably accurate.
Film manufacturers use special standardized lab techniques which include high resolution microscope optics to generate these figures. At Zeiss, we use photographic techniques similar to normal pictorial photography including commonly available cameras with Zeiss camera lenses and achieve the same resolving power figures. So the resolving power, as stated by the film manufacturers under lab conditions, is not just of some theoretical value, but it can be achieved and utilized in real world photography.
As we keep testing new film types as they appear on the market in order to find those that are best capable of recording the rich details that can be imaged with Zeiss lenses, we find a trend among the leading film manufacturers towards higher resolving powers combined with increasing film speed. Today, it is possible to resolve 150 lp/mm with 160 ISO color negative films. In black & white, the same resolution can now be had at a speed level of 400 ISO!
We will report about our findings in future issues of CLN. This will enable CLN readers to make use of today’s sharpest films. | |
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