How to Detect Physical Flaws in a Used Camera Lens
If a camera lens has been abused, mishandled, or is just plain worn out, there are telltale signs that a knowledgeable buyer can look for to help appraise the value or lack thereof in a used camera lens.
The glass of a lens can have numerous problems. Most easily detected are scratches on the exposed glass elements. Sometimes, lighter scratches caused by poor cleaning technique can be present and are more difficult to detect. Bright reflected light is usually sufficient to see “cleaning marks”. Examine the lens with the light reflecting off surfaces at several angles and you should be able to tell if any light scratches are present.
Lens elements are generally “multicoated” with layers of nonreflective optical material. This minimizes light reflection and the resulting lens flare and ghosting associated with the multiple reflective surfaces of these complex optical devices. “Blemishes”, areas in the multicoating where material has been smeared or removed by a bump to the glass, manufacture defect or a solvent splash are less critical flaws provided they are very small (< 1mm diameter) and few in number. A small blemish shouldn't affect image reproduction. Fungus
A lens that has been stored in a dark and moist (and/or humid environment) or stored after getting wet can have a fungus bloom inside the lens. Fuzzy spots and mycelial filaments are both bad news. The fungus can secrete an acid that etches the multicoating of the lens elements, so even if you can disassemble and clean the lens, the damage is likely permanent and will to some degree, affect the sharpness of the lens’ image reproduction.
Examine the lens by peering through it from the lens mount side and look at a bright surface such as an opaque light shade (not the sun!). Open the aperture to ensure you get a good look. If there is anything visible inside the lens, this is not good news. The light path inside the lens should be completely clear of any opacities.
Occasionally, a few tiny dust particles may be noticed when looking into a lens. Especially in older, larger zoom lenses, this is normal and won’t affect lens performance. Consider that the dust will impede about 1/1000% of the total light travelling through your lens, and that the focal point of the lens is anywhere from 12 inches to 6 feet away from the lens. The net effect of these tiny particulates is negligible on image reproduction.
Looking into the front of the lens in bright sunlight will expose the presence of any dust particles. (Don’t look through the lens at the sun, but rather, use the sunlight — or a bright halogen desk light — to illuminate the lens interior as you would use a lamp to read a book.) If you believe a few dust particles will affect lens performance, place a toothpick right in front of a lens while you are looking through your viewfinder — you won’t even notice a difference in the image in the viewfinder, so you can see how a few dust specks won’t affect anything!
When you attach a lens to a camera body, the lens is opened up to its maximum aperture. This gives you a bright viewfinder while you compose your photo. When the shutter is released, the camera allows the lens to stop down to the proper aperture for exposure and then opens the aperture back up so that the viewfinder remains bright, all in the blink of an eye. While the camera actively allows the aperture to close down to the required size during exposure, the action of the aperture is passive in that the aperture movement is “spring-loaded” within the lens to close down to the required size.
When oil is present on the aperture blades, there is friction from the oil’s viscosity and this impedes the quick closing action during exposure. (Normally you would think oil is good for lubrication, but in this case, dry blades move faster.) By the time the aperture has stopped down during exposure (if it even can, depending on the amount of oil present), the shutter action has long since completed, and every photo is overexposed from too much light entering the shutter plane.
Your aperture blades have oil on them if they look like this:
A lens that has been subject to extreme heat can develop an oily aperture. Grease from the focus gear liquifies from the heat and works its way centrally over time into the aperture area where it wreaks havoc. This is a moderately expensive camera shop repair that requires complete disassembly and cleaning, and can double (or more) the cost of lens bargains. Never leave your camera gear in a car in the summer sun, as this is the most common cause of this repair!
The mount should be clean and undamaged, and the gold contacts should be clean and unworn.
Focus and Zoom Rings
The focus ring should turn smoothly, (no significant “bumps” or grinding sounds) with a slightly dampened movement so as to not turn too easily and “creep” when not touched. The focus action of AF lenses by design is not dampened very much to minimize load stress on the AF motor.
The zoom however, should have a relatively dampened feel. Older or heavily used lenses will be a little looser due to wear, and older, heavier push-pull zooms will likely creep in or out when pointed straight up or down. They shouldn’t slide out like a dropping stone, however. A slight loss in dampening action is acceptable and is expected in older lenses, much like the steering of your car is a little sloppier after a few years of use, but you don’t want your zoom collar sliding into the ditch!
About the author: Richard Seagris is an eBay seller specializing in Minolta Maxxum camera gear. Visit his store here. You can also find another version of this article geared towards eBay buyers here.
Image credits: inverted real image by D.H. Parks, Lens fungus by ant217, lens dust! by Neetesh Gupta (neeteshg), Aperture blades on Canon lens by Erwin Bolwidt, EF Mount by 96dpi