Welcome to APUG and thanks for posting this.
However note that the reciprocity failure chart that Ilford provides in the Delta 100 data sheet is not accurate. An indication of the problem is that the reciprocity failure graphs given by Ilford for FP4+, HP5+, Delta 100 and Delta 400 are identical. It is extremely unlikely that these films (which include both conventional and CCG emulsions) have identical reciprocity failure characteristics, so I expect it is sloppiness on Ilford's part copying and pasting the graphs rather than actually performing the measurements.
Both Howard Bond and I have done independent measurements and our results are quite similar, but quite different to Ilford's data! See my post on this at http://www.apug.org/forums/forum37/1...lta-100-a.html. There is a pointer to Howard's results in one of the replies to my post.
My "rule of thumb" based on these measurements is that no correction is required for exposure times up to 4 seconds. For exposures longer than 4 seconds, add an additional 1/3 stop of reciprocity failure correction for each stop of (metered) exposure over 4 seconds.
Last edited by andrew.roos; 01-17-2013 at 03:06 PM. Click to view previous post history.