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The sharpness of a lens is a major concern for most photographers. This might be because we are worried about a defect in the lens, a problem with our technique or a problem with the camera. Or we might want to select a better lens.

In all these cases it is very helpful if we had an accurate and reproducible procedure for checking the sharpness of a lens that can be used by all or most photographers.

So with that background, how can I/should I check the sharpness of my camera lens?

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Related Question: photo.stackexchange.com/questions/2917/… –  chills42 Apr 4 '11 at 13:02

2 Answers 2

I've read the Norman Koren tutorial on the subject a few times and he offers up a test chart as well as some links to others (some are dead, however). In a nutshell, however, you basically need:

  1. A lens test chart. The usual reference is the United States Air Force lens test chart, but there are variations on this and he suggests that it's inappropriate for digital tests and offers up his own for testing.

  2. A solid surface. Wooden and carpeted floors are not good, concrete is good. Basically, you want to eliminate vibration.

  3. A very sturdy tripod, again to eliminate vibration.

  4. Mirror up delay and a, preferrably, a cable release or remote, the remaining vibration equation.

The sweet spot of a lens varies, so you'll probably want to test at various apertures using a constant ISO (preferrably 100), so make sure everything is very well lit. Anyways, reading and handling the results are on his site and I probably shouldn't repeat them here. :)

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Which test chart do you recommend and why? –  labnut Apr 4 '11 at 11:42
    
I would probably go with his, but I'm not an expert. He gives a good description of how to do put it together. Besides, I think the key to it is consistent behaviour, so while a commercially printed chart may be better, you can probably learn a lot from the one he provides with good technique shooting the test. –  John Cavan Apr 4 '11 at 13:01

Another issue can be back- or front-focus. This essentially means that the camera's AF system is not calibrated correctly. The lens may be very sharp but the camera sets the focus incorrectly.

A quick and dirty way to check this is to use a ruler, set at an angle away from the camera. By focusing on a spot on the ruler, you can inspect the image to see if the focus is indeed on that spot, or closer or further away.

There are a number of test charts available for download that make this easier, I used Jeffrey Friedl's chart. Another more expensive setup is the Lens-Align.

Some more higher-end cameras allow you to set an offset for each lens to correct for focusing errors, up to a certain point.

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