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04-16-2010, 02:28 AM   #1
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how do i check a lens for front/back focusing and general sharpness?

i am going to buy my tamron 17-50, and i have heard it is important to check and make sure that your copy is a good one. how do i check for the front and back focusing, and general sharpness?

04-16-2010, 03:21 AM   #2
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General sharpness is fairly easy, Just take some controlled test shots in decent light. If you have a tripod, then set up a still life subject and test the lens at a few different focal lengths and different apertures. If n tripod just put the camera on a solid surface. Use the 2 second mirror lock up.
Then do some of the same (without the timer and SR on) hand held and check the pictures.

Then to check BF/FF, download the focus chart here and follow the instructions exactly. This will tell you what you need to know.
04-16-2010, 05:24 AM   #3
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You could try this - it helped me.
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/94734-advice-required.html
Cheers and happy shooting.
04-16-2010, 09:23 AM   #4
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QuoteOriginally posted by Peter Zack Quote
Then to check BF/FF, download the focus chart here and follow the instructions exactly. This will tell you what you need to know.
Except that in the cases where it does show a problem, it won't really tell you if the problem is with the lens or with the camera without doing further tests (with other lenses of the same focal length and aperture, and with other cameras using the same lens). In general, I believe camera problems are probably far more common, but they tend to go unnoticed when all you have are slow zooms where the DOF is more than big enough to mask any slight focus discrepancy.

On the other hand, the word "problem" is relative term. It's probably normal for all cameras to be off by a millimeter or two, and probably has been since the first AF systems 30-some-odd years ago. But the nature of digital is that everyone's a pixel peeper now, plus it costs nothing to perform the tests, and we can see the results instantly, so everyone's a measurebator too. Discrepancies of a millimeter or two that never would have been noticed ten years ago are now easier to spot. That doesn't mean they actually matter any more than they did before, though.

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