I was just identifying three 8x10 film holders recently with corner and edge light leaks. One was obvious, a large chip on the corner of a dark slide opposite the tab end. The others were hard to determine, but they are at the hinged end of the holder. First, the flexible tape at the end is worn and loose with the film holder body. Second, there is a larger than normal gap between the plastic parts at each end corner. These gaps are on the outer face, at the corners. Age, or who knows why, cause the plastic parts to shrink very slightly. With the loose tape and a slight gap at the corners, light gets to the film corners. I set aside and marked the bad dark slide and the bad film holder, which I won't bother to try to repair. One more failure, A C-41 color 8x10 negative had a big light streak. It was a long crack in the plastic dark slide, hard to notice unless it was removed and flexed to open the crack. It paid off to methodically check the film holders and refer to my field notes.
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