Calculating Reciprocity Failure

[Leonard Metcalf]

I have been using Ilfords reciprocity compensation table to calculate the new exposures and am frustrated by how difficult it is to read. Are there any alternatives to calculating reciprocity times. (any formulas or existing tables?) I was imagining a simple table with common times on it... ie 2, 4, 8, 15, 30, 1 min, 2 min, 4 min etc....

Thanks...

[Stan. Laurenson-Batten]

Hello Len.

This question is a bit of a teaser.

From my personal experience, although there are those that abide by specific formula for calculating the correct exposure where reciprocity has, or likely to occur, it is still a guessimate type of a problem to solve - there being no real substitute for experimentation.
As most LF forum readers will be aware: Exposure is equal to the light intensity that is multiplied by time that the light is acting on the film emulsion. Should the reciprocal system of light level,
emulsion speed and exposure be unbalanced, the film emulsion will not act to the expected formula. This breakdown is know as reciprocity falure. Stan. L-B

[David A. Goldfarb]

Over on APUG there's a thread started by Patrick Gainer on this subject. A search should turn it up. I think the title was something like "reciprocity misbehavior."

Gainer suggests that if you can determine the reciprocity factor at 1 sec. experimentally, you can reliably plot the rest of the curve, and he describes a method for doing so. I haven't tried it myself, but it looked interesting.

[darter]

Steve Simmon's "Using the View Camera" book has exactly the chart you are seeking.

[Andrew O'Neill]

Ilford's factors are way too high. The best way is to run a test yourself. I run an in-camera test using Gordon Hutching's Zone Board as a target. I expose sheets at 1/4, 1/2, 1, 10, 100. For the 10 second and 100 second exosures I use ND filters instead of stopping down the aperture. Develop film, read densities with a densitometre and plot a graph. If you don't have a densitometre, you could try using my data. I've been using it for years.

1/2 sec = 1/6th stop

1 sec = +1/3 stop

10 sec = + 5/6th stop (1.7x)

100 sec = + 2 stops (4x)

Make a graph using this data and you can find inbetween compensation times. HP5+ doesn't need any development compensation for long exposures.

[K. Nicolaisen]

Len

The meters exposure times in seconds raised to power 1.2 for most colour films, and raised to power 1.4 for most b&w films will give you proper exposure times in seconds.

If you don´t use most films, then make tests.