[EDIT – September 2017] – Please note that the reciprocity adjustments featured in this blog post, have been superceded by new information from Ilford – see here
I mentioned in an earlier blog post that I had made some pinhole images using Ilford FP4+ without accurately adjusting for reciprocity failure. My quick fix for pinholes has been to switch to Fuji Acros, which requires no adjustments for exposures of less than two minutes. However I then went off and shot two rolls of FP4+ in dark woodlands, with measured exposures of 1-2 seconds, and failed to take any account of reciprocity failure.
Many of the images also used a green filter, and my estimate of the required extra exposure may have been too low.
So I decided that to avoid forgetting to take account of reciprocity failure, I really needed a handy table which I could carry around to guide me. I went off to read the Ilford data sheet for FP4+ which can be found here
Here is the relevant section:
The problem with this table is that it is too small, and is lacking intermediate gridlines, to enable an accurate assessment to be made of the adjusted exposure time. This is particularly true in the <5 seconds range. We can see that a 5 second measured exposure requires an adjusted exposure time of somewhere in the range of 12 or 13 seconds, but measures of less than 5 seconds require a great deal of eyeballing and guessworking based on a very small chart.
So I used the Microsoft Clipping Tool ™ to capture the graph from a PDF, saved it to a JPEG, inserted the JPEG file into Word, and resized the image to fill a page, making sure to keep the original aspect ratio of the image. This enabled me to draw minor gridlines on a 1 second basis between 1 and 10 seconds and to estimate the following set of adjusted exposure times:
I graphed the resulting numbers to get this chart:
My chart allocates more space on the horizontal axis for the measured times up to 10 seconds, than for the times above ten seconds. My rationale for doing this was that I will frequently use exposures in, say, the 1-8 seconds range, whereas for me exposures of 10+ seconds are commonly only used for pinhole images.
As well as displaying as chart markers the adjusted exposure times for 0.5, 0.7, 1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, and 60 seconds, the graph includes minor grid lines on the horizontal axis at 1 second intervals, so that intermediate values can be read off the graph for a measured exposure time of, say, 12 seconds.
I should now give some caveats. First, although I think I have been able to display some information with a finer degree of granularity than the original chart, there are limitations to the process, based on issues such as the the quality of my eyesight ! In some cases my estimated interpolations give an adjusted exposure time in between a whole second, eg 6.5 seconds; I have rounded these up because I can’t directly enter these onto a camera. In fact with most of the cameras I use I would be relying on “bulb” mode above 8 seconds and there is no way I can accurately time a bulb exposure to half a second.
Therefore, these tables are nothing more than my initial starting point, which I will try out for a few months to see how they work. I make no guarantees whatsoever about their accuracy !
Now I don’t like to leave a blog post without showing some of my photos, so here are a few examples from the last couple of FP4+ rolls. The underexposure is not too obvious in these images, as the scanner has managed to make adjustments, but some of the images have very poor shadow details when viewed larger.
All images were taken with a Fujifilm GA645Zi and developed in Firstcall B&W film developer.