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Tuesday, May 25, 2010

The Certainty Of Sharpness

How to test your lens’ sharpness in the digital era

Labels: Lenses

This Article Features Photo Zoom

An example of light falloff when shooting into the sky. The edges are darker than the center. Canon PowerShot G10 at 1⁄1,000 second, ƒ/5 and ISO 80.
On a recent photo safari, a colleague asked me to review the images he had captured with his brand-new, top-of-the-line DSLR and matching high-end lens. He had obtained the camera the day before he boarded a plane to Africa. We had paid a prime fee for a hot-air balloon trip over the Namib Desert that morning, and every one of his captures from the shoot was out of focus. He suspected the new camera was the culprit. So we improvised some tests of the camera using the fold of a map as the focus point. We determined that the camera’s actual focus was significantly behind the area selected in the autofocus sensors; his aerial views of features on the landscape were actually sharp somewhere underground!

Lesson learned—again. Nature photographers need to be sure their cameras and lenses are working at optimum levels before they head into the field—or the air. If you’re going to invest time, effort and money on any subject, you need to have complete confidence in the camera and lenses in your bag. For years I conducted complex, painstaking comparative lens tests for my journals, The Natural Image and The Digital Image. Now, in the era of highly capable, sophisticated DSLRs, you can quickly and painlessly test the performance of your equipment, make adjustments, if necessary, and work with assurance that your camera/lens combination is at the top of its game.

The LensAlign system set up for focus testing.
Common Lens Problems
What causes a photographer to question his or her camera or lenses? Sharpness is usually the first concern, and the complaint I hear most about. To assess a sharpness problem, eliminate the biggest variable—the photographer—first. If you get even one sharp image from a camera/lens combination, the odds are that any problems with sharpness after that result from improper technique.

Beyond operator error, lens sharpness can be compromised in a number of ways. The optics can be poor or faulty from the outset. Dust, dirt, water spots or smudges on the front or rear elements have greater deleterious impact than you might imagine, or the lens could have sustained damage in transit or during use in the field. The camera body plays a big part in perceptions of lens sharpness if the autofocus functions are faulty. These problems manifest themselves in images that aren’t properly defined, low in contrast, or have the focus either behind the intended subject or in front of it.

Other vexing lens problems include fringing (unwanted color around the edges of details); distortion (horizontal and vertical lines are bent); flare and lack of contrast (resulting in flat and uneven lighting); and vignetting (where the light falls off to the edges of the image). So how do we determine if the optics we’re using are giving us the image quality we expect? Before you experiment, check out the manufacturers’ specifications and the reports of independent lens testers to see how equipment like yours has performed in other assessments. An example of the latter is the Photozone Lens Testing section at www.photozone.de. The information available at this site is very thorough and well presented, but keep in mind that the data presented is specific to the individual lens tested, and yours may vary.


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