Buying or cleaning a used camera lens? 5 Reality Checks You Can Do
There are useful checks you can do before you shell out your hard-earned money for a lemon or use on your own lens before shooting. Lens errors will detract from the image you are so carefully trying to make. No matter how much you spend on a camera, if the lens is substandard or worse, there’s nothing you can do to save the shot once it’s taken. Watching these pointers is a good way to maintain high technical quality.
Fingerprints are killers.
Oily fingerprints contain acids that can leave permanent marks on lenses if left there. They also degrade image contrast, often by as much as 25%. In the center are real killers. Check your lenses often and clean them as soon as you find them. Carefully inspect the used lens for any fingerprints.
A microfiber cloth, slightly shiny, wiped gently in a circular motion without rubbing, will usually remove the stain. If you must go for a chemical-based one, use an alcohol-based lens cleaner. Wipe very gently in circular motions and as little as possible.
Their effect depends on where they are and whether they are fine or coarse. Fine scratches, such as those caused by cleaning with inappropriate materials such as the end of a shirt or whatever is at hand, are usually the worst because they scatter the light on the receptor. Contrast is reduced and often the lens coating is destroyed. It is cheaper to buy a new lens than to recoat the defective one.
Deep scratches, especially near the middle of an item, would be too much of a problem. Leave the lens or get a big discount. Often these spots are evidence of lens abuse and can be a signal of other defects such as misaligned lens elements. One way to minimize this scratch is to use a fine permanent black ink marker and carefully fill in the scratch by wiping off the excess. Black will reduce glare.
A scratch on the back element is worse than that on the front because the back element directs the light straight to the receptor. The footprint of the front element is modified by the other elements before it leaves the lens barrel, and will be outside the depth of field anyway.
External dust can be removed with a soft lens brush. Be careful not to use a tissue or cloth, which can scar the lenses from the particles. I tend to avoid air in a case as I have sometimes seen a sticky substance leave the case and stick to the lens making it look much worse than before.
Internal dust usually comes from peeling off the black anti-reflective coating that occupies the inner walls of the lens. Modern techniques avoid this, but it’s worth checking older lenses. Since black doesn’t scatter light, this isn’t really a problem unless it’s a lot.
Wet conditions can lead to this and cause real problems. It spreads over the elements in ever-expanding strands that, if not removed, will etch patterns into the glass. Like fingerprints, it does a lot of damage to contrast. The only remedy is to have the lens disassembled and cleaned by a repair specialist. You left too late, forget the cleaning and buy a new lens. It’s cheaper.
Excessive cleaning. don’t do it
Too much scrubbing can remove surface coatings that are there to prevent glare and halos. It can also cause flat spots in the carefully designed curvature of the lens, degrading refraction, focus and affecting magnifications.
A good lens test is to take your camera, mount it on a solid support (read my tripod article) and shoot a page of newspaper. Examine it closely for distortion or out-of-focus areas. This can tell you a lot about the lens without expensive test equipment or charts. The best tip for lens care, don’t get them dirty. Always replace the lens cap when not shooting, and consider a UV filter as protection in dusty, wet or windy conditions. A well-maintained lens will give you great service for many years. Happy shooting.
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