Reciprocity and contrast changes

[Marco B]

Hi all,

Through some other thread here on APUG, I got to discover Ralph W. Lambrecht's DarkroomMagic website and some handy technical tools he has available under "Library":

Dead Link Removed

Now this raised a question. I have been shooting quite a lot of nighttime photo's, often at extended exposure times where reciprocity kicks in. In the "Reciprocity compensation table" on Lambrecht's website, he not only shows some recommended compensation, but also a column with "theoretical contrast change". With values upwards of N+3 (that probably means a heightened / bigger contrast, does it?)

I had never realized these were related... Dumb me probably probably, but anyway, does reciprocity failure indeed significantly change contrast, and are their people out there that do change their development practices accordingly? Any recommendations and info appreciated...

Marco
My website

 

[Jim Noel]

Yes, extended exposure definitely increases contrast. A sketchy explanation is the fact that bright areas record light immediately and continue to do so until the darker areas begin to record.

If reciprocity charts indicate a need for up to N-1 the additional contrast is easily taken care of with variable contrast printing paper. If more than N-1 is indicated it is a good idea to follow those guidelines stated in the charts.

 

[Marco B]

Thanks Jim,

This helps a lot.

Marco

 

[Lee L]

In an article on testing reciprocity failure in Photo Techniques magazine in July/Aug 2003, Howard Bond found that modern films have less increase in contrast, and thus less need than older style films for reduced development with the long exposures required to correct for reciprocity failure. TMY and 100 Delta showed no increase in contrast at 4 minutes exposure, and Tri-X only a small increase. The Kodak recommended development reduction for Tri-X of 10%, 20%, or 30% at different extended exposure times didn't fit Bond's findings at all.

I've also seen this reported elsewhere in a more anecdotal fashion, without such rigorous testing as Bond applied.

So I think the best advice with modern emulsions is to test and/or critically examine your results as you work, and not necessarily take conventional wisdom based on older style emulsions as always valid with current films.

Of course many night scenes (especially in cities with artificial lighting) tend to have a much greater dynamic range than typical daylight shooting, which might call for reduced development. But this isn't a case of reciprocity failure increasing effective contrast.

Lee

 

[Marco B]

In a way I agree with you Lee, as I haven't seen any real issues with most of my negatives. Actually, in a sense strive for some extra contrast and deep blacks in most of my night shots, so a small bit of extra contrast might not be a real problem.

 

[dancqu]

Lately I've been shooting Acros for it's total lack
failure out to 120 seconds. Some use the film rather
than a faster due to it being effectively
faster beyond 4 seconds. Dan

 

[Bruce Watson]

Think of it like this. Reciprocity failure normally doesn't happen across the whole film at once. It happens in the shadow areas first.

For example, say you are making a photograph of a stream flowing around rocks. Say it's a stream in the woods -- lots of shadows. So you end up with an exposure of 2 seconds or so. What happens here is the white water flowing around the rocks is properly exposed while the dark shadows caused by the rocks are in reciprocity failure and so are underexposed.

You end up with some choices. You can expose for the shadows and develop normally only to find that your highlights are blown. You can expose for the shadows and use an N- contraction to pull the highlight density back down. Or you can expose for the highlights, develop normally, and let the shadows go black. Etc...

But yes, reciprocity failure can be related to whole negative contrast increases because only part of the negative is seeing reciprocity failure while the rest is forming the latent image normally.

 

[gainer]

See "LIRF is Lurking at Tour F-Stop" at www.unblinkingeye.com by some kook named Gainer.

 

[gainer]

"Tour" should have been "Your" of course. Reminds me of the old story. A notice of a typo appeared in a newspaper:"In the last issue we referred to John Smith as a Defective in the police force. We should have said he is a Detective in the police farce." In our local newspaper, a story covering an overturning of a truck said it took a 1000 pound wench to right it. Quite a gal, that.