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08-17-2009, 08:11 AM   #1
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General Lens sharpness Question-NOOB

I have a Tak 135/2.5 that I've been using on my k20d. I keep reading about how certain lenses are "sharp" at certain f-stops and not so "sharp" at others. Taking pictures this morning at dawn, I primarily kept it at 2.5 with the ASA at 100. Should I have tried other f-stops with an ASA or 200, 400, or? Is sharpness considered clearly focused or?
Any information about this is welcome.

08-17-2009, 09:25 AM   #2
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I take it you aren't satisfied with the results you got, and that's why you are asking? Why virtually no lens is at its sharpest wide open, you also need to think abut depth of field. Google that term if you're unfamiliar with it. Basically, at that focal length and that aperture, you would not expect more than a few inches or so to be in focus, depending on how far away your subject is. So you have to be *very* careful about where you are focusing, and not expect anything more than a few inches in front or behind to be sharp. Stopping down (and raisng ISO if necessary to keep shutter speeds up) is how you increase DOF.

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