Understanding Sharpness

Started Jan 4, 2013 | Discussions thread
drj3 Veteran Member • Posts: 6,203
Re: Understanding Sharpness

I assume that you have already done the equivalent of the coke can test or you would not have asked about this.  I also assume that you have taken photos with the lens and are less than satisfied with the result.  The coke test will only let you know if the coke photo is as sharp as you think it should be, it will not give you any quantifiable measure of sharpness.  I know from personal experience that my wife will think an image is sharp, when I find it totally unacceptable.  If you had two identical lenses and took the series of photos with each, you could probably determine if one set was sharper than the other, and at least know if one of the lenses on that camera was sharper than the other.  However, I doubt that comparison of a wide angle zoom with a standard kit zoom with tell you very much.

Is the problem lack of sharpness of the lens (nothing at any distance from the lens is in focus) or with a front or back focus problem?  If it is the latter problem, then it can be corrected, if you camera allows focus adjustment.  If nothing at any distance is in focus and other lenses on the camera work correctly then the lens has a problem.  The image you provided appears to be okay, but that is difficult to determine without more information about ISO and aperture (the image appears to have been taken at a moderately high ISO).  For a test of the lens, the object (get a test pattern from a camera store) should be very well lighted so that you can use your lowest ISO value with a tripod mounted camera (follow your coke can recommendations about shutter release).  You can compare photos taken with manual focus (live view - magnified) to autofocus to see if the lens is front or back focusing.

I should warn you that if you are serious about this test, it takes a lot of effort and considerable time and equipment to do it well.  You will spend a lot of time looking at images.  I have a telephoto lens with teleconverter which back focuses.  I needed to be able to set up the test target in homogeneous lighting with the center at exactly the same height as the center of the camera lens.  The target must be exactly parallel to the lens in vertical and horizontal planes.  If you are testing a zoom lens, you need to be able to move the target or camera.  I worked at a university at the time I did this which gave me access to the facilities and equipment I needed to calibrate my lens.  However, when I finished calibartion I came out with exactly the same correction that I had estimated by simply taking photographs of distant birds.  My recommendation, take photographs, use the lens and you will rather quickly find out if the lens has a problem.  I personally think it is probably fine.  Why not post some photographs with exposure information for members of the forum to evaluate.

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