Right, I'm getting there. I see that Tb is coerced to be at least Tb0, which means that you can never get a faster shutter speed than is set in the Canon menu. In fact, the best you can get (by choosing "High Jello") is that you slow it entirely with Ta, which would in principle keep the exposure angle constant. However, I notice that if I choose 1/4000 before I start, I can get down to 2 FPS quite nicely with a roughly 1/150 shutter, but if it was strictly proportional, it would be more like 1/250. What started as 2.7° crept up to 4.8°. I want to look at the code: I found it at bitbucket because you quoted it and I googled, but how can I be sure that I'm looking at the source of the version I have in my camera? I don't know how bitbucket handles versioning.