I've found lots of great information and advice here on which legacy lens to choose (conclusion: they're ALL great), but I'm having a harder time finding any tips on evaluating the condition of an individual lens. To that end, I've done some Google searching and compiled some tips which I'll share in this post. Please feel free to dispute or discuss any of this advice and/or add your own. Buying sight unseen, i.e. on eBay or other online classifieds (including those on this site): Evaluate the seller Research the seller's reputation. This goes beyond simply looking at the seller's positive feedback percentage; you need to read the feedback comments. Look for comments like "condition better than expected" which can be a very good sign indicating the seller is conservative in their description of an item. Pay particular attention to the seller's response to any negative feedback. Toolhaus.org offers some very useful tools that assist in evaluating an eBay user's feedback. Be wary of sellers (particularly foreign sellers) with little or no feedback as this could be a tip-off of a scam. The further away the seller is and the more expensive it will be for you to ship the lens back (particularly as a percentage of the cost of the item) the more likely the sale is to be a scam. Since it may cost you quite a bit to return the lens, you may not bother, which is what these scammers are counting on. Research the shipping cost to return the lens, and if it is significant, be particularly vigilant in your evaluation. Insist on the ability to return the lens. Beware of any "as-is" sales. Keep in mind that all purchases through eBay are covered by eBay Buyer Protection which is a sort of arbitration process to help resolve a dispute between buyer and seller. This can be used if the item you receive does not match the item the seller described. While this process can protect you from getting scammed, be aware that it may take a while for you to get a refund. In general, sellers on this forum are more likely to be vetted than are those on eBay. You should view the seller's profile and in addition to checking their feedback score and reviewing any comments from previous buyers, you can check how many posts they've made, whether they support the site and how long they've been on the forum. Note the seller's location, since certain lens afflictions can be more common in certain climates. For example, a lens is more likely to be infected with fungus when it is stored in a warm, damp environment; so if you're purchasing a lens from a tropical or sub-tropical locale, ask specific questions about fungus and perhaps about how the lens was stored. Some more specific tips on dealing with eBay auctions can be found in this post by member Euterpeundersail. Evaluate the lens In cases where the seller does not know what mount the lens is for, you can use Rick Oleson's guide or KEH's three part guide -- part 1 (Canon, Nikon, Pentax & Leica), part 2 (Hasselblad, Minolta, Olympus, Mamiya & Bronica) and part 3 (Contax, Zeiss, Kodak, Fuji, Konica, etc.) -- to determine which adapter you would need to mount the lens on your :43: camera. Research the specific lens model you are considering to determine the model's attributes and any common problems to look for. This can help you decide if, for example, a stiff focus should be something to be concerned about or if it's just a trait of that particular model. This can also help you get a general idea of what the lens should be worth. The more detailed the written description provided by the seller, the better. Not only does this let you know more about the lens, a more thorough description also makes it easier to make a case if the item delivered is not as described. Closely examine any photos of the lens for signs of trouble (as described in detail below). Be extremely skeptical of any listing that does not include at least one high-quality photo of the actual lens being sold (not a "stock" shot). Ask the seller to provide sample photos taken with the lens in the highest resolution possible and analyze these closely. If both lens caps are not included this can be a red flag. Conversely, if the sale includes both caps, any standard hood, the original box and the manual this can be a good sign that the lens was well cared for. Ask the seller specific questions about the lens' condition (see below for what to ask about) and make sure you feel good about the seller's answers and forthrightness. Any response that indicates the seller doesn't know much about the item he is selling should be a sign to proceed with caution. Hold on to the seller's responses to your inquiries as they can be useful in making a claim against the seller if the condition of the lens is not as described. If the price seems "to good to be true" redouble your evaluation of both the seller and item. Hands-on testing: Check the lens body Examine how much dirt and grime is on the outside of the lens body. Dirt on the exterior of the lens is easy to remove (with alcohol or a similar cleaner), but excessive dirt on the exterior can be indicative of a lens that's been treated roughly or stored in a dirty environment; if it's dirty on the outside, you should more closely examine the lens for any other signs of rough usage, particularly dirt/dust inside the lens. Check the outer case for excessive dents and scratches. Outward appearance shouldn't be deciding factor unless you feel that beauty is critical; minor dings, dents and scratches can be ignored or painted over. Shake the lens gently to hear/feel if there are any loose elements. If anything rattles sharply, walk away. A tiny amount of rattle may be existent in some lenses (e.g. Pentax-M 50mm's), try depressing the aperture lever to see if that eliminates the rattle. Avoid any lens that appears to have been dropped, which would generally be evidenced by dents or serious scuff-marks on the lens body. Check the contacts on the lens mount for excessive wear and tear and for scratches indicating rough handling when mounting the lens. If the lens mount is corroded, skip it. Closely examine the filter ring for cracks, dents or other damage (not vital if you don't plan on attaching a filter or screw-on hood, but damage here can indicate rough usage and will impact resale value). Remove any filters that may be attached to the lens and set them aside until you have completed your evaluation of the lens. Check the lens function Check that the focus and zoom rings turn smoothly (feeling and listening in particular for any "bumps" or "grinding" noise or feel) and that their action is not too loose or too tight. Keep in mind that the movement of the focus ring on auto-focus lenses will be less damped than that on manual focus lenses. Validate ability to stop down aperture and that aperture blades operate smoothly. For zooms, check the ability to exercise the entire zoom range. Movement of the zoom should usually be relatively damped. Older, particularly heavier push-pull zooms may exhibit some creep when pointed straight up or straight down, but they should never drop like a stone when tilted. NOTE: This is not a hard-and-fast rule as some older zooms (e.g. the Nikon E Series 75-150mm) were designed with no damping; know the attributes of the model you are looking at. Check that any switches on the lens operate smoothly. Check the lens elements Clean the front and rear elements of the lens using a lens pen or a microfiber cloth. Perform a flashlight test by shining a not-too-bright flashlight through the lens (be careful not too blind yourself by looking into a magnified light). Shine the light from both the front and rear elements. You should be looking at or through relatively clean glass. If the surface or particularly the interior of the lens is fogged, clouded or hazy, skip it. Look for excessive and/or deep scratches or chips on the front and (more importantly) the rear element. Scratches near the lens' periphery will generally not cause degradation of image quality. If the lens has scratches near the center of any element, avoid it. Check for excessive dust inside the lens. A certain amount of dust will be visible (particularly via the flashlight test) in every lens and it will typically have a very minimal impact on image quality, but higher than normal amounts of dust can have an impact and will certainly lower the lens' resale value. Examine the lens for fungus which will appear as a fine mesh-like growth generally starting from the side of the lens barrel toward the center. Fungus may look like fine spider webs or mold spots; in rarer instances it can look like bacteria. It can be difficult to completely eradicate fungus from a lens and attempting to may require disassembly. In addition, fungus secretions are acidic and can eat into the lens and coatings, so eradication of the fungus may not save the lens. Any lens with evidence of fungus should be avoided. Check for significant erosion of lens coatings by looking toward the (cleaned) lens at a bright light or brightly lit wall. This can appear as white spotting. By varying the distance from the lens to your eye you can check each element. Erosion near the edge of the element is more common and less critical, but erosion of the coating near the center of the lens will impact image quality. Check for lens element separation, which is when two cemented lens elements begin to separate. Look through the lens at a bright light or brightly lit wall for any bubbles or patches of discoloration at the element edges which might indicate lens element separation. Separation can appear as an iridescent or rainbow-colored crescent on the edge of the lens element. Separation is difficult (read: expensive) to repair, so if you find evidence of it you should walk away. Check the aperture blades for oil or oily residue which can hamper the action of the blades and lead to your photos being overexposed; dry blades work best. Oily aperture blades may appear as a flower petal pattern when the aperture is fully stopped down (i.e. closed). If you suspect that oily blades may hamper performance, use a toothpick to slide the aperture control rod to stop down the aperture; when the rod is released the aperture should immediately snap shut. Addressing oily aperture blades will require disassembly of the lens. Oily aperture blades are not a major problem with adapted lenses where the the aperture is pre-set since guick movement of the blades is not an issue here. However, if a good deal of oil is present on the blades it could eventually spatter onto lens elements which may cause soft focus spots, lower contrast and exposure issues. Validate that all of the lens elements are in place; you don't want to purchase a lens with a missing rear element. On-camera testing: Check the lens function on camera For autofocus lenses, make sure the autofocus operates smoothly and that the autofocus motor does not generate any unusual or excessive noise. Validate that any switches on the lens function as expected. Check that the lens mounts and un-mounts smoothly and there is no looseness between the lens and camera when mounted. Check the lens output Since lenses are simply tools used to make images, it's the images that can be created that are the true indicator of the lens' value. Ideally you'll want to mount the camera on a tripod for this and all of the following shooting tests to eliminate differences that might be caused by camera movement. In evaluating image output be sure to look at the entire frame, not just at the center of the image. Test for lens flare by shooting at an angle to a bright light source (a window or lamp) which may be at the edge or outside the frame. Note that most older lenses (particularly those that are not "multi-coated") will exhibit some flare; how much is "too much" is fairly subjective, so this test will be most useful when making comparisons between two or more lenses or examples of the same lens. Lens flare can be minimized by shading the front of the lens with a lens hood or even with your hand. Test for image smearing by shooting at a brightly lit object in a dark setting (e.g. a lit building at night). If you note some smearing effect around the pin points of light that may be caused by internal haze. Test for excessive blooming by shooting high contrast subjects with sharp edges against a dark background. While blooming is a digital artifact, some lenses may exhibit it more than others. Check for excessive vignetting, which can be an issue with smaller adapted optics (most notably with small C-mount lenses) which do not cover the entire :43: sensor Test the lens at several different apertures. Most lenses are a bit soft wide open (i.e. at their largest aperture, which corresponds to the smallest f/ number), and achieve their maximum sharpness when stopped down by one or two stops. Shoot an image with a shallow depth-of-field and evaluate the qualities of the bokeh in the out of focus areas. This is highly subjective, but you'll want your images to have a bokeh that is pleasing to you. For autofocus lenses, perform a focus test to determine the accuracy of the focusing mechanism. If the lens has OIS (optical image stabilization), validate this functionality works as expected and also that it can be turned off. If possible, load any sample images you shoot onto a computer and do some "pixel peeping" on a large monitor. Pay particular attention to any chromatic aberration in the images. Ideally, compare the output against a photograph you like that was taken with a similar lens. I recommend a conservative approach to buying used lenses. When in doubt, set it down. There are way too many good used lenses out there to take a chance on a lens you're not sure about. While it's possible to make usable photographs with an apparently flawed lens, it's best to play it safe. Life's too short (and good lenses are too cheap and readily available) to shoot with bad glass. One way to develop an eye for determining a lens' condition is to evaluate the lenses you already own. This can help you get a sense of what to look for, e.g. how much dust inside a lens is "too much". You can also go to a camera shop and ask them to let you look at a new/pristine example of the lens you've got your eye on for comparison. Anything I missed? Any of this advice you'd dispute? Hat tips to the Camera Lens Buying Guide by Adrian Wong on TechARP.com, How To Detect Flaws in Used Camera Lenses on Ebay an eBay Guide by Richard Seagris, How to Detect Physical Flaws in a Used Camera Lens by Richard Seagris on PetaPixel.com, PAWNSHOP LENSES (and other used lenses) - A Buyers' Guide by RioRico on PentaxForums.com, Buying / Testing a Classic Camera by Karen Nakamura on Photoethnography.com, this article on MFLenses.com, a post by forum user Antonis and this thread on the Pentax Forums.