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Basic Lens Testing |
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UPDATE July 16, 2005: A new and updated version of this page can be found here in the new LEARN section of this site. Lens testing can be as simple or complex as you want to make it. If you prefer not to do any testing yourself, there are several sources
for lens tests. Popular Photography
magazine regularly tests lenses. Their results generally agree with my own assessments of lenses which I have used. I consider them to be good general guides to the overall quality of the lenses they test. They will occasionally publish a list of all of the lenses they have tested. For a small charge, they will send you a copy of any lens test they have done.
I rely on the lens tests done by George Lepp in his quarterly journal, The Natural Image. If nature photography is important to you, and also keeping up on equipment and techniques, The Natural Image is worth subscribing to. You can subscribe through Lepps' web site . If you want to test your own lenses, you can start with simple tests. Pick a brick wall with lots of texture and
photograph it when there is good directional side light on the wall. Use a high resolution slide film, put your camera on a tripod and photograph the wall. Keep track of lenses and apertures used so you can
label the slides when you get back. Look at the slides through a loup and ask yourself how sharp the images look. Are the lines straight or do they bow in (pincushion distortion) or bow out (barrel
distortion)? Compare different lenses at the same aperture or the same lens at different apertures. For a more demanding test, tape up a sheet of newpaper want ads with a variety of type sizes. Pick a
distance standard for comparing lenses, like one inch of distance for every mm of lens focal length. If you are testing a 50mm lens, the camera should be 50 inches from the newspaper. With a 200mm lens, the
camera should be 200 inches from the newspaper. When you get your slides back, look through a good loup and ask yourself how small a print the lens can resolve. Compare results to other lenses you own. Which are your sharper lenses? What are the sharpest apertures? In general, most lenses are
sharpest around their middle apertures like f/8. Any aberrations in the lens design show up most at the widest apertures like f/4 for zooms and f/2 or f/2.8 in single focal length lenses. All lenses lose
sharpness at the smaller apertures (f/22 or f/32) due to diffraction (the bending of light rays as they pass through the small aperture opening). Zoom lenses are sharper in the middle of their focal length
range. If you have an 80-200 zoom, it is likely to be sharpest at 135mm, a little less sharp at 80 mm, and the least sharp at 200mm. Telephotos zooms are usually sharper at the short end than the long
end. This means if you have a 28-100mm zoom lens and a 100-300mm zoom lens, the 100-300mm will probably be sharper at 100mm than the 28-100mm is at 100mm. When it
comes to lenses, you get what you pay for - sort of. A $400 zoom lens may be a lot sharper than a $150 zoom lens of the same focal length range. A $1000 zoom lens may be only slightly sharper than the $400
zoom. The $1000 lens usually has a faster maximum aperture (like f/2.8 as opposed to f/4) rather than another big jump in sharpness. If a manufactuere offers three similar lenses in different prices, the
middle priced lens is usually you best value for the dollar, UNLESS you need the faster maximum aperture of the high dollar lens. There are exceptions, of course, but generally these statements hold true.
For a more formal lens testing chart, go to the USAF lens testing
page. There are a number of sources of lens tests on the internet. Photodo has a comprehensive set of lens tests. With their permission, I have
published a |
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Nov 20, 2000 |
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