Dilution and sharpness

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Autonerd Senior Member • Posts: 2,944
Dilution and sharpness

Hey all -- an interesting tidbit came up on a thread in Open Talk (Re. DPReview TV Medium Format Mythbusting) that I thought should be reposted here. KenW wrote in his reply:

Developer "sharpens" a film on a fixed dimensional scale independent of the size of the film. This is caused by the fact that along a bright/dark edge on the film the bright side robs the dark side of developer through molecular diffusion. The result being the bright edge ends up a little brighter than the center of a bright patch (because the bright edge could "steal" some unused developer from its neighboring dark patch) and the dark edge ends up a little darker than the center of the dark patch (because it has even less developer than the center of the bright patch since the its neighboring bright patch "stole" some). The result is the same as applying sharpening in digital post processing. And of course invert the sense of "bright" and "dark" in the above description depending on whether you are working with negatives or positives.

This process was very well understood by past darkroom masters. If you wanted a "sharper" negative you used a more dilute developer (by making the development time longer there is more type for developer to diffuse across the bright/dark edges and enhance the sharpening effect) and you reduced or even eliminated agitation (because of course agitation remixes the developer across the bright/dark edges reducing the sharpening effect). If you wanted a "smooth" negative use a faster developer with more agitation.

The key point is that this developer based sharpening effect has a fixed physical dimension down to the properties of diffusion in the developer (including dilution and agitation scheme). If you put a 110 negative or a 4x5 negative in the developer you got essentially the same "sharpening radius" in say mm regardless of the size of the film. Relative the to the final print size that mean the 110 negative had a huge "sharpening radius" to borrow a digital term compared to the 4x5 negative.

In digital if we had this same developer sharpening it would be like taking a 20 MP m43 image and applying a sharpening radius of 2.5 pixels compared to taking a 20MP MF image and applying a sharpening radius of 1.0 pixels. We all know how "crunchy" too large a sharpening radius can make an image look. Well, in film the developer was doing that to us all the time when we shot smaller format sizes.

So in film if you moved to a larger format size not only did you get the obvious improvement in grain but you also controlled the sometimes over the top sharpening effects of the developer that on small formats made it nearly impossible to make the same nice subtle "tonal gradations". This developer effect was always visible regardless of the ISO or grain of the film. It was one of the reasons for the "magic" of a larger format in film. But the effect does not exist in digital at all. So it is not surprising that when we shoot "equivalent" images in digital we can't tell what format they came from, while in film there was still a visible cue - that developer sharpening scale.

I asked KenW for his permission to re-post here. He agreed, very modestly adding, "It is certainly nothing particularly original on my part. It is described in a lot of books on film and developers. I'm fairly certain Ansel Adams discusses it in his "The Negative" though it has been many years since I read that one. I've got a lot of film process books in a box in the basement somewhere and if I recall correctly most of them mention it in one form or another."

Proof that I don't read enough books -- this was new and interesting to me and thought it might be new and interesting to some of you as well.

Aaron

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