Response to Whoops, half this negative is overexposed

I do wonder if flash sync was indeed the problem, but this might be because I misunderstood something. Therefore, I would appreciate to read a follow-up. What makes me wonder is this:

As a kid, I made the same mistake myself. I used a flash and had my shutter speed set to 1/125 with the fastest permissible sync time being 1/60. The result was not a negative half of which was over-exposed and the other half being OK, but one that was half unexposed and half OK. This is plausible, too, because as was pointed out, the flash is way too fast for any mechanical element (shutter curtains) to follow it. Thus, when you use an electronic flash, it must fire when the full image window is open. To my knowledge, the fastest sync time (given in your camera manual, and sometimes marked somehow on the camera) is that speed where there is still one instant when the leading shutter curtain has passed the window, and the second one has not started following it yet. At all speeds higher than this, the second curtain starts before the leading one has reached the end its sweep, so there is no such fully open window, and at any instant during the exposure time, only a slit of the negative is exposed. That in mind, I cannot see how part of the film should be *over*-exposed. I could understand *under*-exposure when flash bulbs are used, because these are slow, and there may still be some light before and after the optimum point, but an electronic flash is usually so fast that there should be light when the shutter is fully open and no light at all when the shutter is (partially) closed. (That is why you can use electronic flashes to freeze fast movements. You can't use bulbs for this purpose.)

On second thought, I could think of one way to achieve over-exposure: If the available light was actually sufficient for the correctly exposed part of the negative, the additional light from the flash effective for the other part might cause the over-exposure. In that case, however, using the flash was probably not necessary.

I have read that most focal-plane shutters have vertical slits nowadays, i.e. the shutter curtains run from left to right or vice versa, but not from top to bottom or vice versa. As a first check, find out if the split in negative exposure is parallel to the shutter slit. It should, however, be a shutter problem if it is limited to individual shots on a film that is otherwise OK.

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