Buying a Used Lens—How To Win
Once upon a time, I bought new lenses. Long story short, I don't anymore. You see, people are fickle and camera manufacturers know this. I swear that Nikon and Canon meet secretly to discuss who will have the upper hand for the next 5 years. People jump ship and sell their stuff at a loss to fund the switch. Then there are the people who build up a set of high quality zooms and then decide that they want to shoot primes or vice-versa. I did an internship in Shanghai where I lived 20 minutes away from a 5-storey mega mall dedicated to nothing but photo equipment. Three of those floors were dedicated to used items. There's definitely a huge market for used lenses in great condition, at bargain prices. This guide will hopefully arm you with the knowledge and just the right amount of paranoia to separate the gems from the dogs.
I've always taken highly technical lens reviews with a grain of salt. There's too much in terms of production variation, and I'm talking about new lenses here. So when you're testing, you should find the right balance of scrutiny. There are two extremes here. On the one hand, you can give the lens a cursory glance for dust and damage, take a few test shots, and be on your way. On the other hand, you can show up with an MTF chart, optical collimator, and a microscope. Your testing will take hours, the seller will tap his/her foot impatiently and probably end up storming out in a rage. I guess what I'm saying about used lenses, if I'm saying anything, is that you should know all the features and specs but you shouldn't expect the same result as what DxOMark publishes. You should expect some margin of error, which means you don't have to do super precise testing. Having said that, I'm now going to detail the things I do when I test a lens.
Check the barrel for any signs of abuse. Scratches in the paint or bent filter rings are usually signs that the lens has been scraped or dropped. Gently twist, pull, and push different sections of the lens to reveal anything loose. Every time you notice a flaw, ask the seller about how it happened.
Switches and Mechanisms
Depending on what type of lens you're buying, it may have have a switch for AF, IS/VR, IS/VR mode, focus distance limiter, focus lock, etc. Make sure that they work properly. The lens may also have a ring mechanism for focusing, zooming, de-focus control, aperture control, etc. Turn them back and forth and check for any issues like dirt or sand. Zoom and focus are usually smooth, the aperture dial usually goes click-click-click. If the lens extends during zooming or focusing, check for creep. If the lens includes a tripod collar, make sure that you can tighten/loosen/remove it.
The Two Ends
On the side of the front element, try mounting a filter to make sure the thread isn't damaged. On the rear element side, check the screw holes to make sure none of them are missing. Check for corrosion on the contacts. If your lens has a rear gasket for weather-sealing, check to make sure that it isn't damaged.
The Glass Bits
On the outside, check for any scratches or damaged coatings. Depending on the lens, a scratch may range from being a non-issue to being very problematic. In general, fast telephotos are more forgiving compared to ultra-wides. A scratch or defect on the rear element is a much bigger deal than a problem with the front element. Now point the lens towards a light source and take a look through the lens. All lenses have some internal dust—even new lenses. What you're looking for is stuff like cracked internal elements and any sort of debris. If you're buying a really old lens, check for fungus.
Mount and unmount the lens a few times. Make sure that it seats firmly and doesn't jiggle about.
Switch the camera to A/Av mode and set the aperture to wide open. Now hold down the DoF preview button while slowly stopping down to make sure the lens responds to each aperture setting. Unless you're buying a Leica lens, don't worry about the blades not creating a perfectly symmetrical opening at smaller apertures.
Autofocus on a few things. Make sure that the lens sounds like it's supposed to. For example, an AF-S/USM lens should be nearly silent. If your lens has IS/VR/OS/Voodoo/Witchcraft, make sure that they work properly. Take a few test shots with it on and off and note how much slower you can push your shutter while retaining a sharp image.
I knew lens caps were useful for something! Focus on a single letter on a lens cap—across several points in the zoom range if you're using a zoom lens, and take a few wide-open shots to mitigate any user error. If you're the guy on the extreme who brought 20lb of testing gear, I'm sure you'll have a LensCal handy. Of course, if you're that person, you wouldn't be reading this guide. Review the images to make sure that the letter you focused on is the sharpest thing in the frame. If it's slightly off and you have a body capable of MFA, then you're ok. Now stop down a bit and repeat. Note any unusual focus shift. For example, lenses with floating elements shouldn't focus shift. Also note any unusual optical abberations that might suggest manufacturing defects or misalignments due to abuse. For example an image of a flat surface with one side of the frame sharper than the other could mean a misaligned element. Don't mistake field curvature for misalignment though, know the lens you're buying!
"Sharpness is a bourgeois concept" - Henri Cartier Bresson
Again, don't get caught up with matching your particular copy of the lens what reviewers publish. Instead, look at the lens. Look at the results. Is the build quality up to scratch? Are the images the results that you want? Does this all justify the expense? Is the wear and tear and lack of warranty worth the money you're saving? These are the pertinent questions. We all have different standards, so I leave these things to your judgement.