When buying a used lens, what should the buyer check? Other than looking for obvious physical damage on the outside of the lens, what else should be inspected to identify the condition or quality of the equipment?
I have a checklist. It mostly applies to buying classic (manual-focus) gear to use, but is probably of general use.
It's a good idea to try all of these on your own gear first, since you know how you've treated it, and hence what effect you've had, and what "normal" gear feels and looks like when you're really looking closely.
The Most Important Step
Know what you're after.
Do a bit of research on the lens you want. Classifieds/eBay/KEH/etc will give you an idea of the price, and camera forums will give you an idea of common problems to check for with specific models. Both of those are probably more useful than most of the stuff below. If you're going to buy a lens on a whim, make sure it's for an amount of money you don't mind losing!
For example, something like Stephen Gandy's guide to Leica M lenses will tell you a lot more about what to look for than any general guide.
With that in mind, here's my general steps for checking out a lens:
0. Keep your perspective
Remember you're not an expert, not a collector, and that your evaluation is subjective. If you want to use a lens rather than keep it on your shelf to resell it later, most problems are actually pretty minor. You would be shocked at the quality images you can get from the ugliest-looking gear.
1. Look at the glass
2. Look at the aperture
3. Fiddle with the mechanics
From the bad experiences I've had you have to look at external signs of abuse first. If the lens is too dented, then it's a sign that the owner didn't treat it well.
Then there's the fungus that can build up inside the lens if it's not taken care of properly over time. You have to look through the lens against a bright light looking for black spots that give away fungus build up.
Another thing that almost got me was the auto-focus. Make sure you twist the zoom ring and force the auto-focus motors many times all the way through. One lens I looked at was slow to auto-focus and it made a very low screeching sound when it got to the extreme focus. It turned out the user had banged it and some parts were touching inside.
Other than that I think it has to do with how much confidence you have about the seller.
Shamelessly stolen from PentaxForum's RicoRico, but may be of interest: http://www.pentaxforums.com/forums/pentax-lens-articles/59245-pawnshop-lenses-other-used-lenses-buyers-guide.html
BRIEFLY: The quick-and-dirty approach.
I walk into the slightly seedy premises, wander over to the photo gear display, look over the Pentaxian offerings. I discard focal lengths I already have too many of. I pick up each lens, judge its build quality, twist and slide any moving parts, push any pins and switches, look into and thru it for cruddiness, see if the diaphragm blades move. I pull a M42-PK adapter ring out of my pocket, see if the lens mount looks/is compatible. Then I might offer ten bucks (US$10) for it. Or maybe not.
But this method won't work for everyone, so here's a detailed screening technique.
INTRO: Basics of lenses and mounts.
Lots of lenses will fit your (and my) K20D. Many will have some sort of PK mount, the bayonet twist-on mount like the lens that (hopefully) came with your camera. Many more will have a 42mm diameter threaded screw mount, known as an M42. To use an M42 lens, you'll need an adapter. I strongly recommend an honest-to-goodness PENTAX-brand adapter, to avoid all sorts of grief, both in focusing, and in removing the adapter. (I won't go into fitting other lens mounts onto a Pentax camera.)
Some of either sort of lens, PK or M42, may say PENTAX or PK on them somewhere, and some won't. Besides all the more-or-less prominent third-party makers and branders whom I won't list here, you'll also see some fine lenses from Sears and Ricoh. Some Ricoh-made lenses are branded as Sears. WARNING: a Ricoh lens with an RP designation on it has an extra pin that WILL jam the lens on your camera. Other Ricoh lenses are fine, just avoid the RP's.
And of course the best lenses say Asahi and/or Pentax and/or (Super) Takumar. Some that say Takumar Bayonet are not highly regarded, so don't be talked into paying a lot for those. Some older Pentax-made lenses are branded as Sears or Honeywell. I won't go into the pros-and-cons of other brands, except to mention the Russians - see the warning below on Russian lenses.
SCREENING: What to run away from.
When stalking pawnshops for lenses, be sure you have 1) your camera, 2) a PENTAX-brand M42-PK adapter, 3) a small flashlight, not too bright, and 4) a dust pen or lens brush. (Maybe mud-over the camera so you don't look too rich.)
Gross appearance shouldn't be a deciding factor unless you think beauty is critical. I bought a Vivitar 90/2.8 macro whose knurled rubber grab was decayed. I finally peeled it off and replaced it with duct tape. The lens, one of my favorites, with crystalline optics, cost me US$3; a 'cherry' version might cost 50-100x more. Minor dings, dents, scratches can be ignored or painted over. (Tell the girls you were a combat photographer.) Remove any filthy filters before assessing the lens. Major damage should be avoided - like, it should all be and stay in one piece. If it falls apart when handled, skip it.
To examine a lens, first shake it a little. If anything rattles sharply, skip it. Then try turning the focus and aperture rings. If they're too tight or loose, skip it. Make sure the diaphragm leaf blades open and close when turning the aperture ring. You may have to push the stop-down pin (if any) and/or move the M/A switch (if any). If blades or pin or switch don't move, skip it. If the lens mount is corroded, skip it. If you cut yourself on anything, skip it.
Now use the lens pen or brush to whisk away lens dust, and whip out that not-too-bright flashlight and look inside the lens, from both ends, shining the light both into and thru the lens. (You don't want to blind yourself when looking into magnified light.) It can be pretty scary in there! If the lens surface or interior is fogged, clouded, or just cruddy, skip it. If the glass is scratched anywhere near the center, skip it. Minor scratches around the periphery may not matter much, but they certainly lower the value. Dust inside the lens may be a problem; if you see much dust, skip it. You want to be looking at and thru rather clear, clean glass. If not, skip it.
Pointing the light into the lens, work the aperture. Look at the diaphragm blades. Are they clean, or oily? If oily, skip it. Check from both ends. NOTE: To get the most from this and all the above screening tests, you should go to a camera store and ask to inspect a new lens and/or superior used lens. This will give you an idea of just what a good lens' condition is, and thus what a prospective used lens should aspire to.
Other aperture tricks: some older and simpler lenses have no stop-down pin. The aperture can only be set from the ring, called a pre-set. Some lenses have two rings, on that lets you pre-set the smallest aperture you'll want, the other to actually (and smoothly) dial in the stop-down. Make sure the rings are easy to turn, not too tight or loose. And make sure they actually stop-down the diaphragm. If you have a problem here, skip it. And some modern auto-aperture lenses have NO aperture ring; with such, you can't check it at all, until trying it on the camera. I wouldn't trust one of these unless the pawnshop or seller guarantees that you can return it after purchase. Yup, that includes our fine Pentax DA lenses. Consider them guilty until proved innocent.
If the lens passes so far, check that the mount will actually fit on your camera or the M42 adapter. Don't force it - if it doesn't fit right on, skip it. WARNING: Some Russian lenses branded Helios and Jupiter and Industar (or anything with Cyrilllic characters) have 39mm threads (M39), not M42. Some of those M39s, fitted with a cheap adapter, will work fine on your camera. Most won't. And other lenses made for Leicas and their clones have M39 threads and WILL NOT WORK! If it doesn't thread smoothly onto the M42 adapter, skip it.
TEST-FIRING: Trying-out the candidate.
You will NOT be able to fine-check the optical quality of the lens now. Well, you could fire off some shots, then review and pixel-peep, but it's hard to judge quality on the little LCD screen. The best you can probably do is check the lens' mechanical operation, see if it actually works on the camera. WARNING: Unless the shop/seller explicitly states a guaranteed return policy, or the lens is REALLY REALLY CHEAP, you should ALWAYS try it out before purchase. I've failed at this a couple times. I then repeat my mantra: OWAH TAGU SIAM. Say it over and over.
FOCUS: Does the lens manually focus? Look at something and try. The K20D's focus-lock lights, or the ZX-M's split focus screen, should help here. After getting it in focus, maybe look at the lens' distance scale and see if it's about right. Does it focus to infinity? Does it focus as close as the scale says? If it's an autofocus lens, does it actually autofocus? Is it loud, grinding, slow, hunting with futility? If focus isn't good, skip it.
APERTURE - PRESET: A lens with no stop-down pin and/or no M/A switch and/or no aperture-ring A-setting, will likely only work in Av mode. (Setting the mode to anything but B or M won't matter - both the K20D and the ZX-M will default to Av.) Set the lens M/A switch (if any) to M. Focus on something, half-press the shutter, note the shutter speed. Now twist the aperture ring a bit and repeat. Does the speed change? It should increase when you open up, decrease when you stop down. If not, skip it.
APERTURE - MANUAL: Otherwise, set the lens M/A switch to A, and the aperture ring to anything but A (if available). Set the K20D to M (Manual) mode. Focus on something. Hit the Green button. The lens should audibly stop down as the camera takes an exposure reading. Note the reading. Now manually change the aperture and repeat. Does the reading change, as with presets? If not, skip it, unless 1) the lens passes the PRESET test and 2) you'll be happy using it as preset-only and 3) it's cheap.
APERTURE - AUTOMATIC: Ah, the blessed PKA mount! Set the aperture ring to A. (If there's no A there, skip this test.) Set the K20D to P mode. Focus on something, half-press the shutter, note the reading. Try turning the front and/or rear e-dials. Do the aperture and speed change? If not, skip it.
ZOOM: Is it a push-pull or twist'em or power zoom? Does it push-pull, twist, and/or power smoothly, quietly, effortlessly? Can you tolerate the zoom creep, if any? Does is seem to zoom from minimum to maximum of its range? If not, skip it, unless it's one of those huge obnoxious lenses and you just want it to show off. Camera bling, yeah sure. If it's a macro-zoom (or any kind of allegedly 'macro' lens) does it reach its close focus? If not, skip it, unless you just don't care.
OUTRO: Closing the deal.
That's all I can think of right now. This should be sufficient info on what to look for in a used lens that you can fondle before purchasing. It's your credit card and/or financial future; go for it!
I'll mention that pawnshops near major casinos are great places to look for used photo gear. Winners buy a new, upgraded outfit; losers go home naked. Or so I hear. My favorite pawnshop in a major Nevada city (I won't say which) has a great assortment, and sends a Sweet Young Thang out to show lenses to male customers. (I won't mention her name.) She volunteers that she's done some modeling. I suspect that she's rather effective in diverting some customers' attention from prices and other bothersome details. So yes, self-discipline is needed in this kind of shopping. Be strong, be wary, and don't spend more than you can afford. Yeah, right.
My advice would be to buy from a reputable dealer like KEH; you pay a bit extra, but you also get a warranty and a no-questions-asked return period. For me, that's a good tradeoff.
Besure to examine the aperature opening/closing correctly. If you are looking at a telephoto lens, look for fluidity in zooming in/out.