Thinking about purchasing a used lens for your digital SLR? It can be a risky game. Sure, the price is lower than retail, but you also have to deal with the wear and tear the previous owner put the lens through. Because none of has the time to test every little detail on a used lens, I’ve broken it down into an easy to follow guide. If you are looking to purchase a used lens, make sure you do these tests first.
Check for problems with images taken with the lens
This is the first thing I check. Sometimes the outside of a lens looks fine, but the optics inside have been damaged by a fall. The quickest way to test is to take a couple of test shots. It helps to take a photo of an object with a straight line (like a fence, or a wall). If the lens is slightly out of alignment, these straight lines may not be as straight on the image. Take these shots while directly in front of the the object to minimise any distortion that can come from a perspective shot.
Check for scratches or small dents on the outside of the lens
You first want to find out what kind of abuse this lens has been put through. Has it been dropped before? A scratch or small dent on the exterior is a good sign the lens has been dropped. If you notice either of these, ask the seller how the damage happened. Anything you can do to get more information will help you make a better purchasing decision.
Twist the lens and flip the switches on the sides
Check to see if the switches perform their functions, and make sure you do it using your own camera body. Twist the zoom and focus rings to see if there is any resistance. Sometimes sand gets caught in these parts of the lens, resulting in a slow kind of wear and tear on the internals. If you hear any weird noises, take note of them and ask the lens owner what may have caused them.
Test the front and back lens connectors
Take the lens off of the camera it is mounted to, and look for any scratches on the underside where it connects to the camera body. Also take a close peek at the gaskets and note any rust or corrosion present. Sometimes water gets into these connections, and it’s all downhill from there. Make sure you bring a U.V. filter with you so you can test the threads on the front of the lens. If it’s relatively easy to attach filters, that’s a sign that the lens is still in good condition (i.e. it wasn’t dropped, causing dents on the threads).
Mount the lens to your camera
I always make sure I mount and unmount the lens to my camera. I do this a few times, checking the strength of the connection while listening for any sounds that might be out of the ordinary. A good lens produces a noticeably audible “click” sound when it is attached to the camera body.
Check the focus/autofocus capability
Mount the lens to your own digital SLR, and play around with the autofocus. Note how quickly it responds, and whether there are any strange noises (noticing a theme here?). Autofocus should make a slight noise, but once it becomes long a drawling hissy sort of noise, there’s likely an issue with the AF mechanism. Compare the noise to your other lenses if you need a reference.
At this point, you will also want to test the accuracy of your autofocusing system. Find any object with protruding text, and focus on that (your lens cap is a good one as you always have it on you). Next, focus on a letter and take a picture. Look at it closely in review mode to see if there are any strange looking color bands (a.k.a. chromatic aberration) or other oddities like bent areas. Each of these reduces the value of the lens.
Check the aperture settings
This one is a little challenging, especially if you don’t know what you’re looking for. The electronic aperture on a lens is the little hole that light travels through to make an image on the sensor. Every time you adjust the aperture up or down, this hole shrinks and grows. To see if it’s working properly, you need to switch over to your camera’s “depth of field preview” mode and then watch as your aperture adjusts itself for each setting (not all camera models have a depth of field preview mode. Consult your camera owners manual or online documentation to find out if yours does).
Those of you looking to purchase a strictly manual lens will have a much easier time. You only need to look at the aperture while adjusting the settings on the lens itself.
Lastly, when you purchase used lenses, understand that there’s really no need to be paranoid about every detail. If you’re getting a bargain on something that would have otherwise cost you a fortune brand new, just go for it! There is such a thing as “good enough,” even when you make your living as a photographer.
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