I recently developed two 35mm films together in a tank. One, and only one of them, came out with artefacts on most (but not all) of the exposures – spots of uneven development that seem to originate from the film sprocket holes. Here's one frame from the roll for illustration:
(Negative digitized using a digital camera; you can just about discern the film sprocket holes behind the improvised white mask.)
I'm new to film processing, having developed ~10 films so far; I have never encountered this problem before, but it looks to me like an agitation issue. The procedure was as follows:
- Loaded two 35mm films in a Jobo tank, one reel on top of the other. The films were Ilford FP4+ rated at EI64 and Fomapan 100 rated at EI100. The film with the problem is FP4+; Fomapan came out perfect. I don't remember which film was on top.
- Developed in 1:4 Ilfotec DD-X for 8 minutes in 20°C, following Ilford's recommended agitation routine (4 inversions spread out over 10 seconds every 1 minute), except that at one point I had a bit of a lapse of attention and there was 1½ minutes between two successive agitations (and then only ½ a minute before the next one).
- Stopped in Ilfostop.
- Fixed with 1:4 Ilford Rapid Fixer, 2 or 3 minutes.
- Washed using Ilford's "fill tank with water, invert n number of times, pour water out, repeat, repeat and repeat" method.
My specific questions:
- What caused this artefact, and why does it appear on one film only?
- Would the order in which the films were loaded have an effect? Is the damaged one more likely to have sat at the bottom or at the top?
- What can I do to avoid this sort of thing in the future?