We use a the Zeiss modified Siemens Star Chart. Star Charts are often used as focusing aids, which is one of the reasons we put them on the resolution charts we use for Imatest and our other testing setups. But it’s at least 95% accurate for detecting decentering in our experience (which is for several thousand lenses tested over-and-over). The screening test I’m going to describe is not perfect: a few lenses (particularly ultra-wide and 10x zooms) will give false-positive results and this test won’t detect other causes of softness like problems with spacing of elements. You need just a couple of accessories: a tripod to give your camera a stable platform and a simple chart. But if you just want to check and see if your lens is centered properly (at least for most lenses) you don’t need much equipment at all. Now that we have live-view focusing and the ability to look at images in real-time, we nearly have the same thing as a centering collimator built into our camera and lens. If you want to correct a decentered lens you need an optical bench, a computerized MTF program, or at the very least a lens projector and a lot of knowledge about which elements can be adjusted to correct an abnormality. (You can find them on eBay every so often if you want one to keep around the house.) A Simple Test for Decentering Standard centering collimators became a thing of the past, except for some specialty shops. 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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