The equipment is breathtakingly expensive and only the factory and some (not all) factory authorized centers have access to it. When lenses became more automated, so did testing: Lenses are mounted to the manufacturer’s electronic test system and most of the adjustments made electronically – or the computer report suggests which lens elements need be adjusted. Obviously in film days you didn’t take a test shot, send it off to be developed, make an adjustment, take another shot. The technician would then adjust those elements that could be adjusted until the lens was properly centered.
If an element was decentered the chart would flare or be distorted in one direction. It shined a star chart or a chart of concentric circles through the lens. The Way It Used to Beīack in the days of film and manual focus lenses, most repair shops had a centering collimator. But if the lens doesn’t have a reputation for soft corners, it may well be that the copy in question is decentered. Sometimes that’s just how the lens is designed. But one common issue people ask me about is a lens that seems OK in the center but is very soft in the corners.
Unfortunately lenses are too complex for that. It would be nice if we could say “a decentered lens looks like this” and “a tilted lens causes that”. Lens with a Poor Spacing of a Central Element
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