Sample Graph of Reciprocity Failure for Ilford Films

As previously posted on Ag+ Process, the paper on reciprocity failure for Ilford films contains formulas for computing exposure times for long exposures. The longer the exposure, the less likely a given film will perform according to its box speed. For those with a hand calculator or calculator on a phone that has a y^x key, the computation is easy and just requires looking up a constant.

However, for those who prefer graphs, Ag+ Process has prepared a sample graph from the Ilford data for the first 30 seconds of measured (e.g. with a light meter) exposure time. It is possible to see that for a graph with longer times, reading the computed (Tc) times for shorter exposures becomes harder. Other factors can come into play as well for longer exposures.

To use the graph, find the curve for your film (they are colour coded and all but two films are labelled), then find the time along the horizontal axis (labelled Tm) from your exposure meter. Trace up vertically to the point on the curve for your film exactly above that time, then trace horizontally to the vertical axis (Tc). That is the time for the corrected exposure.

Here is the graph, click for full size: