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09-21-2013, 08:12 PM   #1
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How to test second hand AF lens?

I'm familiar with RioRico's post on Pawn Shop lenses, and my goal here is not to re-invent all the info he accumulated there. My specific question:
If you have a chance to buy a second hand AF lens in person, how might you test to see if the AF is working well? I've seen a number of discussions of lenses that had quality issues during production, where getting a good copy was hit and miss, and the lens I'm interested in seems to fit this description. If all you have to test with is your camera, how can you be sure the focus is sharp when all you can do is zoom in using the LCD on the back.

Suggestions of any sort most welcome.

09-21-2013, 08:28 PM   #2
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For screwdrive lenses: I don't think I've ever had one fail. But I had one that was sticky through a range, but it still worked.
For SDM lenses: Roll the dice. Also listen for squeaks. SDM failures occasionally show first signs of failure by requiring a warm-up period. If you have a 16-50 or 17-70 you can pretty much pen the epitaph, IMHO.
09-21-2013, 08:40 PM   #3
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For screwdrive lenses make sure you check that there isn't too much friction in any of the rings.

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09-21-2013, 08:46 PM   #4
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You have two separate issues:
1) Does it work mechanically. As noted above screw drives rarely fail and SDM, well who knows. To test I would turn the lens to minimum focus and then AF on something relatively far away. Repeatedly. If it consistently locks focus, I think that's all you can do. On SDM if there are any failures or sluggishness I would be concerned.

2) Does it lock focus at the right place. In other words is the image resulting from a focus lock sharp and actually in focus. To test properly you need a good target, a tripod, good light and repeated testing. Not sure you can do that correctly in a casual test at a shop. I would try to shoot some images of a subject with good detail at several distances. And if possible take a tablet or laptop along, shoot jpeg and check on the big screen.

Front or back focus is not necessarily a reason to not buy if it is consistent and you have a camera that allows fine tuning.

I tested a DA*300 at a shop this morning. It was missing the hood and had a pretty good mark on the barrel, I assume the hood took the brunt of whatever caused the mark and suffered for it. I turned the lens to minimum focus and then focused on a sign across the shop 5 times. Focus was slow, though it eventually locked in. I looked at the images and everything seemed in focus but something just did not feel right so I passed. Maybe it was fine but gut feelings rarely lead me wrong.

Hope that helps.

09-21-2013, 11:04 PM   #5
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QuoteOriginally posted by jatrax Quote
1) Does it work mechanically.
I would add that the resistance should be low and the same lock-to-lock with the lens off-camera or with AF disengaged.


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