Just for the sake of completeness, we should mention that pre-exposure is a useful tool for transparency films. It was the only way to tame contrast a bit on the now-discontinued Kodachrome (which was what I used it for back in the day) and is still a viable technique for chromes. Of course, pre-exposing transparency film affects the highlights, not the shadows, yielding more detail, but less contrast.
For negative materials, it is less useful, but can yield good results with films like TMY; it turns its straight, steep toe into a gentler, curving slope, more like 320Tri-X. For those that shoot roll film, I would think that pre-exposure would be used more often for the occasional contrasty scene with lots of deep shadows, since the frame is going to get over-developed anyway (i.e., not receive an N- development, but get N with the rest of the roll).
Best,
Doremus
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