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Lens chart

Last week I purchased a used Canon EF 28-80mm f/2.8-4 L lens from Ffordes. As usual the first thing I did was check it for sharpness at various zoom settings and apertures. I usually do this by pinning a newspaper sheet on the wall and setting the camera on a tripod and checking that it's square on and level to the sheet. Then with the mirror lock on and remote release, I take a number of centre and edge shots and check them on a PC monitor.

It's not very scientific, but what do you compare the results to and how do you know what you should