Tuesday, June 9, 2009
Hi-Tech Studio: Exact Focus
Even high-end lenses can exhibit some slight front and back focus. A simple device can have you dialed in for perfection.
If you frequent Internet photography forums, you might read gallant tales of searching for, and ultimately finding, a “good copy” of a particular lens. I’ve heard of people trying six or seven lenses in hope of finding one with perfect focus. Frankly, the odds of finding a camera and lens with a combined, measured deviation of zero is pretty slim.
Now, there’s a simpler way to make every lens you have in your photographic arsenal a keeper. LensAlign PRO from RawWorkflow.com, the company that created the popular WhiBal series of white-balancing cards, allows the discerning photographer to test, measure and identify focus accuracy with his or her lenses.
Michael Tapes, the inventor of the LensAlign, conceptualized this device even before camera manufacturers enabled in-camera fine-tuning of lenses. Tapes created the device as a way to make life easier for both consumers and manufacturers to identify when a camera or lens was out of alignment in a quantifiable way. His theory was that if you had an out-of-kilter lens, you could include a photo of the LensAlign result to the manufacturer with the lens. Then, when you receive your lens back from repair, you could repeat the test to see if the problem was resolved or not. And now that Nikon, Canon, Olympus, Pentax and Sony all offer the AF Fine-Tuning feature on their latest D-SLRs, the LensAlign becomes that much more useful. If you have one of these D-SLRs, you can save the time and hassle of sending your equipment in for service. Instead, you have the ability to measure and adjust your own gear.
The device itself is very well thought out, with a focus on maintaining accuracy and simplicity. The main pieces are constructed from extruded PVC, which won’t warp or bend due to changes in humidity. The engraved measurement ruler is made of die-cut steel, and the glossy finish ensures a high-contrast image for evaluation. The front and rear sighting pieces are cut from the same die to eliminate variability, and each unit is tested on a laser jig to ensure perfect alignment of the center sighting holes. This attention to detail is necessary. “A measuring device is only as good as its degree of precision,” Tapes explains, adding, “and we’re pretty serious about precision around here.”
The face of the LensAlign is essentially a giant focusing target, with nine high-contrast black and white circular patches of converging lines in a three-by-three grid. The middle and bottom rows of targets have small holes in their centers. A few inches behind the front face is a second vertical piece that has bull’s-eye targets with red center dots and very small centered holes. When everything is lined up properly, you can see the red dot in the middle front sighting hole through your camera. As a double-check, you can look from the back side through the small rear sighting hole and see your camera lens centered in the front sight. This design enables you to get the face of the LensAlign exactly parallel to your camera’s sensor, which is crucial for taking a meaningful measurement.
The measurement ruler magnetically attaches at its zero axis, which corresponds to the front of the focus target. You can choose from five different ruler angles. The more upright angles show more depth of field and the shallower settings minimize depth of field. Depending on the focal length and shooting distance, you may want to try a few different settings to see which one works the best. Tapes recommends, and I agree, to use settings 3 and 4 as a starting point.
The first step in using LensAlign successfully is to get everything squared up. I put the LensAlign on a tripod and then put my test camera on another tripod of similar height. To make life a little easier, I used a Manfrotto hot-shoe bubble level to level and plumb both the camera and the LensAlign. Tapes explained to me that this step isn’t really necessary, but I figured it couldn’t hurt and doing so satisfied my inner perfectionist. The distance from your camera to the target should be roughly 25 times the focal length of the lens, which translates to about eight feet for every 100mm of focal length.
Next, I used Live View mode and zoomed in on the center sighting hole. With a few moves of the tripod and some small tweaks to the ballhead, I was able to line up the red dot in the middle of the center sighting hole. This step takes literally about one minute. It was quick and easy.
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