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I've read that you can examine a lens for scratches and defects by holding the aperture ring at its lowest setting and shining a flashlight through the lens.

When I do this, however, a lot of stuff becomes visible, but the lens seems to take perfect photos. Even what appears to be scratches don't show up in the final product.

Its all well and good to say "Test each used lens before you buy it", but its not always possible to do that, especially when at camera shows. (By test, I mean take a picture and look at it on a computer screen, not just the LCD).

So, how do I better interpret what I see on the flashlight test, or is there a better way of examining the glass?

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4 Answers 4

up vote 10 down vote accepted

All lenses will show dust with a flashlight shown through them, even brand new ones.

Here are a couple of detailed articles on how to evaluate a used lens, from Calvin Foo and TechARP.

Finally, scratches matter less than you might think. Check out this lens.

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looks like the Calvin Foo link is pointing to a spam website right now –  akram May 4 '12 at 4:01
    
Thanks for the heads-up. Not spam; rather, they got hacked. If it's not fixed soon (or if anyone has evidence it's been unrepaired for a long time), I'll remove the link. –  Reid May 4 '12 at 19:27

The first thing is to buy used from reputable sellers. Many Buy and Sell forums have a feedback system which allows you to make an informed decision.

The next step is to do your homework. Used lens hold their value, unless badly mistreated, or made obsolete (in some cases this can increase the value). If the lens is way off what you find on B&S Forums or ebay prices, then it's either stolen, or problematic.

Fungus is a huge no-no, and I prefer my lenses to come from smoke free homes (hard to verify).

Dust inside the element is okay, but front element scratches (while not likely to cause light-path issues) is a sign of a mistreated lens, and I would immediately walk away from purchasing, unless sold at a considerable discount.

Check the focus rings for smooth motion. There shouldn't be any grinding, or grit. Push-pull zooms, and non-constant length zooms may have some barrel creep, but if it slides too quickly with no force, I would consider not purchasing.

Minor focus issues can be corrected with a trip to Canon Service, and even with the service fee, you may be left with a better deal than had you purchased a 9+ lens.

Ensure the lens caps still fit snug, and the mount ring is clean. If you can mount the lens, and ensure the AF communication works (as well trying out focusing).

As I stated, buying from a reputable reseller will mean that you have a good recourse should something not be to your liking.

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1  
You can smell the lens, because cigarette smoke leaves a pungent residue that you can pick up easily if you hold your nose to it. +1 on the fungus. You can check by pointing the lens to a bight sky a peering into it from very close in the back. –  Dave Van den Eynde Mar 14 at 13:08

Personally, I just try just taking photos of a nice smooth surface (white paper for example) at the smallest possible aperture. Similar to sensor dust, scratches won't be apparent at the larger apertures (small f numbers).

Of course, that all depends on whether or not it's possible to use the lens.

That only covers scratches though. A used lens may have other faults in terms of focussing for example if it has been dropped.

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Used lenses lose their value fast if they have scratches, on the barrel but especially on the surface of the front or back lens. Those scratches may not show up in pictures, but that's the psychology of buying and selling used gear.

I would spend more attention on other features. Check if the AF works properly, if the lens doesn't wiggle too much when you attach it, and if the zoom ring or focusing ring turn smoothly.

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I disagree, cosmetic scratches rarely do anything to the value of a lens. –  Alan Jul 18 '10 at 5:49

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