Just out of curiousity, would a low iso film also reveal lens deficiencies? say Iso 6 or something very slow?
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Both the lens and the film (or sensor) will reveal the deficiencies of each other. The image resolution (or system resolution) is always a product principally of lens performance (at a given MTF) and sensor/film performance at a given MTF, plus any other variables that might affect resolution.
The basic formula for system resolution involving the two main variables of lens and film performance is 1/S=1/F+1/L where S,F,L are system resolution, film resolution and lens resolution.
So, for example, if we are interested in how fine the image detail will be that has
not lost more than 50% of its original contrast, we need the resolution of the lens at 50% MTF and the resolving capability of the film at 50% MTF.
A fine-grained B&W film might be able to record 100 lp/mm with a loss of 50% contrast. A good lens like the Canon 50/1.4 would probably be capable of resolving only 60 lp/mm with a 50% loss of contrast.
In this case we could say that the film is
out-resolving the lens, but that does not mean that the film will therefore be able to faithfully record everything the lens delivers.
If we apply the formula to these figures, we get 1/S=1/100+1/60.
If my maths is correct, S equals just 37.5 lp/mm, significantly less than the separate performances of either film or lens at 50% MTF. Nevertheless, the weak link in this example is the lens. If we were to double the resolution of the lens at 50% MTF we would get a bigger increase in system resolution than we would if we were to double the resolution that the film can record at 50% MTF, whilst keeping lens resolution the same.
For example, if 1/S=1/100+1/120, then S=54.5 lp/mm.
If 1/S=1/200+1/60, then S=46 lp/mm. That's still an improvement, however. Despite the fact that the lens is the weak link, we have still increased image resolution by improving the resolving power of the film. (I don't think it would be possible to produce a lens that could deliver 120 lp/mm at 50% MTF.)