I blogged some time ago about creating a table and graph for the reciprocity adjustments for Ilford FP4+, which give some finer gradations than the table included in Ilford’s data sheet.
Since then Ilford have updated their data sheets. Instead of providing the same graph for all of their films, they now produce an equation for each film. The new data sheet can be found here.
The equation is expressed as the measured exposure time, raised to the power of 1.26 for FP4+. If you are using another Ilford film, check the link in the previous paragraph.
Now the calculator on my phone doesn’t have the key necessary to perform that calculation so I decided to update the chart and table from my previous blog post. Here’s the table:
You can see that the new equation gives significantly shorter exposure times than the old graph suggested – which is good news, unless you specifically want long exposure effects.
The graph below gives a visual indication of the differences between the old and new methods:
Of course, Ilford’s equation should only be considered a starting point – you might prefer longer or shorter exposure times to get the effect you want for a particular image or to match with the characteristics of a favourite developer.
Finally, here’s the table I actually carry around with me which gives the adjustments needed for the range of films that are in my freezer right now: