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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

Examples of two resolution charts that can be used by photographers for lens testing. These can be printed on a high-res printer or purchased online for extreme high-res versions that are fairly pricey. The larger chart is the ISO 12233 and the inset is chart USAF 1951, which needs to be printed out multiple times and arranged on stiff board to make a full lens chart.
Color Fringing. Chromatic aberration is another term for color fringing. This problem occurs when the colors captured by the lens don’t focus at the same distance on the sensor, causing magenta or green edging. If this is minor, you probably won’t even see it. If the fringing is extreme, or if you plan to greatly enlarge the image, then you must deal with the problem. While the technology and glass used in today’s lenses minimize fringing, when it does occur, it easily can be corrected in your imaging software.

Adobe Camera Raw (ACR) has a menu under Lens Corrections that allows the photographer to move sliders to Fix Red/Cyan Fringe and Fix Blue/Yellow Fringe. Enlarge the image in ACR. Lightroom 2 and the new Lightroom 3 beta version also address the fringe by using the Develop module for Chromatic Aberration.

Distortion. This aberration is readily apparent if you notice bowing or convergence of sets of parallel horizontal or vertical lines. The problem is more prominent with wide-angle lenses vs. telephoto optics. There’s some improvement as you stop down the lens, but you should check the Photozone website to see what’s expected with specific optics. Some corrections of distortion are possible with imaging software such as Photoshop and DxO Optics Pro, which not only corrects distortion, but also tackles color fringing.

Flare And Lack Of Contrast. If the results from your lens are very flat, with little contrast, you may have a defective lens, but most likely it’s a result of poor technique. Keep the front and rear lens elements clean, and always use a lens hood. Flare is a result of direct sunlight entering the front of the lens and then bouncing around inside the lens. Today’s glass and special coatings do a lot to prevent these situations.

Light Falloff. Many lens designs have a minor flaw where the brightness is greater in the center than out to the edges. Usually, you won’t notice this phenomenon when vegetation fills the frame, but it’s obvious in a prominent sky, and more likely with wide-angle lenses than longer focal lengths. Photograph a multiple-image panorama with sky included and any problem with light falloff will be evident; the composited image will show alternating bands of lightness and darkness in the sky. Help has arrived in newer cameras with a built-in feature that eliminates the light falloff with each lens. If you don’t have one of these latest DSLRs, you can correct the falloff using imaging software like ACR, Lightroom and DxO Optics Pro, to name a few.

Be Vigilant
Like most technical aspects of photography, you can become so concerned about your lens quality that you lose sight of your creative purpose, so you need to achieve some balance. Of all the problems related to lens performance, sharpness is by far the most critical. A standard part of my editing process for every shoot is evaluating the results in the context of my expectations for the equipment I used under the conditions and environment of the capture. If I suspect a problem, I’ll do a field test and check the resulting images on my computer monitor; then I’ll make my own adjustments or send the equipment for repair, if needed. But more often than not, the cause of unsharp captures is technique—in the field or even when conducting camera/lens tests. So while you should be ever vigilant about keeping your equipment in top-notch shape and evaluating its performance, you should apply even greater effort to finding great subjects, satisfying your creative vision and perfecting your technique in the field.


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