Tuesday, May 25, 2010
The Certainty Of Sharpness
How to test your lens’ sharpness in the digital era
Let’s look at each lens problem—what it means to your photography, how to recognize and test for it, and what can be done to solve or minimize it. In the days of film, the medium was the limiting factor on sharpness. Today, it’s your lens. It will be sharpest in the center and less sharp out to the edges. As you stop down your lens, it will get sharper to about ƒ/8 and ƒ/11; if you test it at ƒ/16 and ƒ/22, you’ll find that the sharpness falls off. If you need the depth of field offered by the smaller apertures, you may find the compromise on sharpness is acceptable. Wide-angle lenses tend to have the most light falloff (vignetting) at the edges. As you stop the lens down, the light will be more consistent. And it bears repeating: If you don’t use good technique, even the best of lenses won’t give sharp results. These are the givens that must be kept in mind as you evaluate your equipment.
Resolution And Sharpness. A trip or photo session can be completely ruined when you bring back unsharp results. If this is happening with a new lens or camera body, the culprit may be faulty equipment that needs replacement. It’s better to know this before you invest time in an important subject, so I always “field-test” new equipment before I head out on a shoot. For a simple test, set up a tripod and photograph a highly detailed subject across the whole frame and in the same plane. Use a low ISO for the most detail possible. Be sure the tripod is sturdy, a reasonably short shutter speed is employed, and shoot with several ƒ-stops from wide-open to stopped down (ƒ/16). Using a cable release can’t hurt, and lock up the camera mirror if possible. On the computer, inspect the images at 1:1 (100%) to evaluate sharpness and contrast. When testing a new body, pick a trusted lens for the test. If there’s a problem, it will be revealed under these conditions.
You can undertake more formal tests with a resolution chart. The same charts we used in the film days work fine now—sharp is sharp, no matter the medium. The test targets have patterns of fine detail in the center and at each edge. You photograph them at different ƒ-stops in good light from a tripod, using a low ISO and a cable release to eliminate any external degrading factors. Examine the images on your computer monitor at magnifications up to 1:1, then compare these findings with the results shown on a website like Photozone.
www.imatest.com. Their charts run around $250, and a basic testing kit with evaluation software (Windows only) can be had for $99; their high-end testing systems run from $200 to thousands. For self-testing, charts such as the ISO 12233 are available online from www.graphics.cornell.edu/~westin/misc/res-chart.html. You can also make your own testing chart using the USAF 1951 chart with components found at jimdoty.com. Use a high-resolution printer and gloss or semi-gloss paper to print your chart.
The bottom line is that for any test, you want to photograph very small, verifiable details that can be enlarged and evaluated on your computer screen. You’ll need to consider how your lens resolves these details both in the center and out to the edges of the capture.
Autofocus Combinations. As was the situation with my friend’s DSLR in Africa, unsharp images result when the autofocus function of the lens/body combination is inaccurate. This is especially noticeable with long telephotos that are often used wide-open. When you have a very narrow depth of field, you want it on the animal’s eye and not somewhere at the back of the head or on the nose. Narrow depth of field also poses challenges with medium telephotos with wide apertures, such as an 85mm ƒ/1.2, and macro lenses. If this is your problem, you’ll get sharp images, but the sharpness will consistently lie in front of or behind the intended subject. This inaccuracy can be resolved in many new DSLRs that offer micro adjustments for autofocus that you can accomplish yourself, even to the point that you can program the camera to automatically make different adjustments for each lens you use.
A way to check your autofocus is with LensAlign, an ingenious system for testing the critical focus of your lens. Set up your camera/lens combination on a tripod located 25 times the focal length of the lens (8 feet per 100mm) away from the calibration system, then autofocus on the LensAlign target. If your camera body has Live View capability, you can view the calibration ruler and make camera adjustments for each lens directly. Otherwise, view the image on your monitor and then make the necessary micro adjustments at the camera. Get more information at www.lensalign.com.
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