Light Leak?

I have been seeing this sort of thing, light struck areas at the bottom of the frame, intermittently on some of my negatives for some time now. Checking through my notes, I find that it is not correlated with any camera, film stock, bulk loaded or factory cassettes, developing tank, etc. The only constant is that if it appears on any frame of a roll of negatives it appears on a number of the other frames, at least to some extent. The above is a frame from a bulk loaded 24-exposure roll of HP5 Plus in a Snap-Cap cassette, shot with a Nikon F6 and developed with Rodinal (1+50). The last roll of film I shot with that camera, the last roll of film I loaded in that cassette, and the last time I followed that developing regimen showed no such issues. So what went wrong this time?

A fundamental maxim of good experimental procedure is to not change more than one variable at a time in a series of experiments. But I wanted to try a new film and developer combination anyway so I loaded the same film (HP5 Plus) in a different cassette (a Leica FILCA) and shot it in a different camera (a Leica IIIf) and used a different developer (Ilfosol 3). It was in no way a proper follow-up of the roll that produced the above image but it may still have given me a lead to an answer.

I always start to develop a roll of film by getting together the film cassette, the developing tank, a pair of blunt nosed scissors and a changing bag. I pull enough film out of the cassette to cut the end off square and clip the corners to make it easier to start the film on the reel (Hewes stainless). I open the bottom of the bag and put the developing tank, the film cassette and the scissors inside. I close the bottom of the bag and then, as I always do in cool weather I remove my sweatshirt for a better light seal of the changing bag sleeves in my bare arms, take off my Apple Watch, and put my arms through the bag sleeves to start loading the film. But this time, for some reason, I stopped and asked myself if I was sure I took off the watch the last time I developed a roll of film. And I distinctly remembered that I did not put it back on afterwards.

The face of the Apple Watch lights up spontaneously when I move my arm and rotate my wrist. A simple test of miming the loading actions outside the changing bag showed that the watch turned on and off a number of times. Was this enough light to cause the above effect? And, if so, why is the light leak always at the bottom of the frame? I’ll follow up in the next post with a look at a frame from the latest roll. I think it is interesting in a couple of respects.