What To Check For when Examining Used Lenses
There has been a fair amount of discussion recently regarding used lenses and what to check for when examining them. I thought it might be useful to go thorugh the list of issues we've seen in used lenses.
- Sticky diaphragm. I've seen this on a variety of lenses, both MF and AF. You can spot it by flicking the diaphragm lever on the back of the lens. The diaphragm should open and close very quickly and without any sluggishness. Also, look for oil on the diaphragm blades. This is often the precursor to the diaphragm getting sticky. The natural state of a Nikon diaphragm is stopped down when the lens is removed. Note that other brands may work differently. For example, an Olympus OM lens opens up when removed from the camera. Ease of repairing: easy.
- Inability to focus to infinity or an incorrect infinity mark. I've seen this several times and on a wide variety of lenses. In some cases, the lens couldn't quite achieve infinity focus, but could focus on something that was 70-80 feet away. In other cases, the lens could focus on infinity, but the infinity point was well beyond the infinity mark. While you can get sharp pictures with such a lens, the distance markings and the depth of field indicators are incorrect. This is a little more complicated to fix, but not too big of deal. It's common with AF lenses and with ED MF lenses for the lens to be able to focus beyond infinity. Don't worry if you see that characteristic in your lens.
- Sloppy zooming action. This occurs primarily with older one-touch zooms, the most common example being the old 80-200mm 4.5. While this doesn't affect the optical performance, it can be a nuisance. One trick you can use to improve the situation is to apply several layers of transparent tape to the lens barrel in the area where the zoom ring slides. When you have the right number of layers, the zoom ring will still be easy to move, but won't move on its own. The tape trick is easy, fixing the lens so it's tight again is usually not economically viable.
- Lens fungus. This shows up frequently on lenses used in humid climates. You can see it by opening up the diaphragm, holding the lens to a light, and looking for growths on the lens elements that appear like faint spiderwebs. This is generally the kiss of death for lenses. Depending on how widely spread the fungus is, you can see a substantial loss of contrast.
- Dented filter rings. This one is pretty obvious. If you don't plan to use filters on the lens, it's no big deal, but otherwise it's quite inconvenient. Sometimes a good repair person can straighten out the filter ring and make it usable, sometimes not.
- Inaccurate transfer of f-stop information. I've seen at least one AF lens that incorrectly transferred f-stop information to the body. In the case I saw, the difference was two f-stops. This isn't too hard to fix.
- Small scratches or chips on front and back lens elements. If these aren't too big, it's not a problem and you can ignore them. Try to get a discounted price, however. Major cleaning marks can reduce contrast and increase flare.
- Dust inside the lens. Almost every lens has some dust on the internal lens elements. Generally speaking, don't worry about it. If the lens is really filthy you can have a loss of contrast, but I haven't seen many that have that much dust.
- Focusing and zoom rings that bind. This can occur on both AF and MF lenses even though their internal mechanisms are very different. A good clean, lube, and adjust will generally fix the problem unless there is damage to the mechanism.
Incidentally, I think used lenses are pretty low risk purchases, especially if you know what can go wrong and how to spot it. Good hunting.