Buying used lens: what to look for ?

[sid]

I'm looking at buying a Canon lens from eaby but I'm a little wary about buying a lens without seeing it in person. So, if I do end up buying one regardless, whats the best way to "test" the lens once I get it ? Apart form obvious cosmetic probelms, are there any other things to watch for ?
Any comments from folks who have bought lenses off of ebay ? Or any other sources for that matter.
Thanks !

[Webarcher]

Buying a lens without testing is always a risk, I think. But if the seller has still the bill you can tell how old the lens is. Maybe this is a small insurance.

[rebel61021]

I have gotten a few of my lens from e-bay the one thing I look at is there feedback and if they have sold lens before with good results. Also make sure you do your home work becasue I have found that lens on e-bay only go about a fraction cheaper then what you can buy them new for with a warrenty. Remeber it is a buyer beware and be smart risk you take to save a little money.

[Icecamp]

I have never purchased a lens from ebay however. If I were to purchase a lens(this goes for anything I purchase on ebay)from ebay I won't deal with anyone with less than 98% feedback. I would write the seller and ask if they would allow you to return the lens if not satisfied, I have done this with the two lenses I sold on ebay. Like mentioned above, do your homework, many lenses on Ebay are really close in price to online stores. There are many retailers selling camera equipment on ebay, I use the same criteria when deciding to purchase from them.
I have purchased all of my used lenses from KEH. They offer 14 days to return the lens, which gives you some time to test it. I guess shooting a newspaper would be the best way to tell focus across the frame, and performance at all apertures. Just make sure you have enough light, not enough light is the culprit in a lot of focus problems.
Age has nothing to do with lens quality, my favorite lens is at least 15 years old, the magic drainpipe 80-200 f/2.8L. There is a way to tell the age of Canon lenses, I have forgotten exactly how but it's in the code on the mount side of the lens, I think a letter stands for a particular year.
Cosmetics are the least of your problems. You can consider small imperfections in the front element mostly cosmetic, however the price should reflect this. Imperfections in the front element won't usually show up in the image. The rear element being very close to the focal plane should be in perfect condition. Imperfections there will show up. Brassing, scratches in the finish, dents, etc. won't affect performance at all unless they are excessive. This is a matter of personal taste, however again it should be reflected in the price. KEH is a good site to use as a blue book for lens prices.

[Andy_T]

Brassing, scratches in the finish, dents, etc. won't affect performance at all unless they are excessive.

I agree, however I would be afraid that the impact that got the scratch/dent/whatever in the lens in the first place, might also have played havoc with the lens alignment and interiors.

Best regards,
Andy