Testing for Reciprocity Failure

[neil poulsen]

What are accepted methods for testing for reciprocity failure? Obviously, one wants to consider the deviation from expected exposure for extended exposure times.

I can see where one would want to use a densitometer to measure film density as one increases exposure and look at departure from linearity in some fashion. But, I don't understand the details.

Also, sometimes adjustments in development, as well as adjustments in aperture are recommended. How does one evaluate for both changes in exposure and changes in contrast due to long exposures?

Sometimes adjustments in exposure times are given, instead of adjustments in aperture. How does one determine these adjustments in times?

I'm particular interested in details. Does anyone know good references for testing and for determining corrections for reciprocity failure?

[David A. Goldfarb]

If you're using the standard zone system tests to determine film speed and development time at normal exposure times, you can do the same with long exposure times. So, if you normally determine film speed by photographing an evenly illuminated surface with the lens focused at infinity and placing it on Zone I with an exposure of whatever f:stop at, say 1/30 sec, and then seeing what exposure gives you a density of 0.1 over film base + fog, just do the same with an exposure of, say 1 sec or 10 sec or 100 sec to plot the film speed curve with long exposures.

The development time test would be no different from the normal development time test--see what development time gives you your target density for Zone VIII for your printing process.

[Ralph Barker]

Once having determined "short" exposure film speed and "normal" development, wouldn't you need to do a series of exposures for each time increment, varying the f-stop, develop "normally", and then check density to see which f-stop gives the 0.1 over FB+F (extrapolating the [adjusted] film speed from the f-stop)?

It would also seem to me that one might want to use closer time increments (e.g. 1sec, 10 sec, 20 sec, 30 sec, etc.) to determine the curve, rather than assuming the variation in speed is linear from say 10 sec to 100 sec, or beyond.

Naturally, that procedure would consume a fair amount of film, if using one sheet for each exposure. Film consumption might be reduced by using a series of dark slides with narrow widows in progressive positions, so you could use one sheet for each time increment.

[Eric Woodbury]

This was a good article:

Using Howard Bond's b&w reciprocity data for long exposures from the July/Aug '03 issue, author devises a mathematical formula to determine if a pattern is evident. Law of Reciprocity studied; logarithm, measuring density, H&D curve. Volume 24, Number 5, Page 27, Year 2003

Darkroom Techniques Magazine

[Kirk Keyes]

Check out this site: http://home.earthlink.net/~kitathome/LunarLight/moonlight_gallery/technique/reciprocity.htm

There's a lot of info there.

What I've done is contact a step wedge with the film under the enlarger at 1 second and then by using a combination of stopping down the lens and adding neutral density on the color head determine a new, longer exposure for a second contact. Compare the two contacts with densitometer.

I can force the second exposure out to 256 seconds this way without too much problem. I do have to mask off the enlarger head some to minimize light leaks. I also have a color correction filter on the enlarger lens to daylight balance the light. I used a Minolta Flashmeter IV with a flat diffuser to measure the change in exposure (in the ExIN mode) caused by the f/stop changes and the neutral density amounts that were added to determine the proper exposure.

I've found that Fuji Acros has very, very little reciprocity failure out to 256 seconds - much, much better than 100TMX, 320TXP, FP4+ or Bergger 200..