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I have no clue about the process but if I had to guess I imagine it has something to do with the chemicals not being evenly distributed either by inexperience, some sort of mask used before chemical exposure to achieve the effect, or the process is just prone to doing that.

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The process is just prone to doing that.

It's a combination of things - uneven application of the likely hand applied chemical coating, handling the plates by the edges while loading and processing, unevenness of the metal frame surrounding the plate in the camera, older lenses which had vignetting and lower sharpness at the edges, processing chemicals seeping into the edges of the coating, etc etc.

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