Buying used lenses can be a great way to save money on your most important camera equipment, but if you are not careful you can end up buying someone’s problem lens. Here are some tips on how to buy a used lens.
Visually inspect the lens
You will want to visually look at the glass and the overall condition of the lens before money swaps hands. Start with the front element, look at it at various angles to see if there are any scratches, nicks or other imperfections in the glass. A scratch, no matter how minor, can rear it’s ugly head if the sun is in your pictures. Be sure to also check the rear element for scratches as well.
Look through the lens to see if there is any signs of moisture, fungus or other imperfections. If a lens has been in a humid environment, moisture can get inbetween the glass elements and will lead to the formation of fungus Fungus will block light and affect sharpness of the lens.
It is not uncommon to see some dust inside the lens, and a small amount of dust will not affect your images, but if the inner lenses look hazy, pass on the lens.
Check the focus and zoom rings for smoothness. Both should turn easily and smoothly. If the action feels like it is hanging up anywhere in the movement this is a sign that something internally is out of place or broken (or on the verge of breaking).
Test the Aperture Mechanism
On Nikon and Pentax lenses, there is a little lever that controls the aperture. You should be able to move this lever easily and it should spring back to the fully stopped down position. On Canon, Olympus and Sony lenses the aperture is electronically controlled so you won’t be able to perform this test. Instead, mount the lens to your camera, and press the depth of field preview button. The viewfinder should get progressively darker as the lens is stopped down. If the viewfinder is not getting darker then it is possible the aperture is not functioning properly. If you don’t have a Depth of Field preview button, then set the camera to the manual exposure mode and take sample images, changing only the aperture for each shot and ensure that the image gets darker as the aperture is stopped down, and brighter as the aperture is opened up. Be sure to test stopping down and opening up the aperture.
Test the auto focus
Mount the lens to your camera and press the shutter button halfway to activate the auto focus. Most modern lenses should focus quietly and quickly. Testing the autofocus goes hand in hand with the next test as well.
Test the sharpness
Mount the camera to a tripod and turn off the image stabilization. use mirror lock up (if available) and a remote release. Take several pictures at f/8 and zoom in to see if they are reasonably sharp (not all lenses will be perfectly sharp, but most will be “sharp enough”).
Test the Image Stabilizer
Turn off the Image stabilization and zoom in to the lens maximum zoom. Look through the viewfinder and notice that the viewfinder is rather shaky, the longer the focal length, the more pronounced the effect will be. Now turn on the stabilizer and repeat the test. you should see a significant improvement and may notice a very low pitched hum or whirring sound. The sound is normal, but if the sound is too loud it could be an indication of a problem.
Taking the time to inspect the lens will help to ensure you get a great deal and not a fixer upper. Unless the price is right, skip on the fixer uppers as lens repairs can be very costly. In most cases a repair can cost more than you will save, so be extra careful and be sure to only purchase for a reputable person or store where you can test the lens prior to purchase or the seller provides a return policy. Good Luck!