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04-09-2008, 11:11 PM   #1
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Help: general tips to test for good/bad lens copy

Hi!

A (rather lame, sorry ) question to the gurus: how do I test if I have a "good" copy of a lens or a "bad" one? I'm talking about some general tips and approaches, what to keep an eye on etc...
How to test focusing, sharpness etc.?

Right now I own only 18-55 kit & AF 50 1.4 I didn't hear any complains about those but I'm gonna buy some more and I just want to be prepared.

Thanks!!!
Cheers,
Michael.

04-10-2008, 06:34 AM   #2
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Good question!

Another vote for some general rules!
04-10-2008, 07:00 AM   #3
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Couple of links that might get you started:

Focus Testing - photo.net

Lens testing

There was another site with chart that did the 45deg test to determine if the lens had front or back focus problems but I was unable to locate it now.
04-10-2008, 08:43 AM   #4
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Personally, I think a straight, flat newspaper shot would reveal most the uglinesses of a lens: soft corners, FF/BF, center defect, curvature of field, and distortion, etc. That's what they do with resolution test and distortion test as well (shooting the MTF chart instead). Focus test chart at 45 deg is used when you want to quantify the amount of BF or FF that you have. Besides, I think it's more difficult to angle 45 deg at the chart than just simply shoot perpendicular to the chart. If you want to do focus test chart, just google "focus test chart." There are at least three main ones out there.

04-10-2008, 09:13 AM   #5
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I recently had to send a FA * 24mm back because it was a bad copy. Everything looked good until I started using it wide open. The lens was severely flawed at 2.8 and 2.0. It wouldn't focus at all and had a very strange bokeh. Even with manual focus (and I am very good a manual focus) the images were severly distorted and oof.
Here is what I do:
Visual check: Use the aperture acutator on the back of the lens to open it all the way and check for dust/fungus/scratches etc.
If the lens has an aperture ring, check to make sure it moves freely in both directions, but with good 'clicks' for each f stop. If it has an "A" setting make sure it locks firmly (I had a lens that would slip out of the "A" setting)
I agree with aegisphan above about a straight flat newspaper shot, but then again I do a LOT of manual focusing so AF performance isn't as important to me as it is to others. If AF performance IS important to you I'd suggest to do the 45 degree test, but realize that the lens may be a bit off either way. The AF sensors are larger than the graduations on the chart.
Take the lens out for a shoot. Shoot the way you normally do, but make sure that you use most of the fstops the lens has, paying special attention to the widest and narrowest fstops. Most lenses are at the least OK in the middle, it's at either end that the problems occur. Since I shoot frequently in lower light situations, I pay more attention to the widest fstops. Take some shots of tree limbs against the sky and other known "PF" and "CA" types of shots. Anything with high contrast areas are good for this. Lenses generally perform worse in this regard the wider they are so be sure to open up the lens a bit. Check for flare by shooting more or less towards the sun's direction. WARNING! DO NOT SHOOT DIRECTLY INTO THE SUN!!! Be sure to use a hood. If you are testing a zoom, do all of the above at or near the long end, the wide end and near the middle. You don't need to shoot a bundle of shots, 10 to 20 for a prime and about double that for a zoom should give you a pretty good idea of a lens' performance.

NaCl(anyway that's what I do)H2O
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