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.
While you can just shoot a test picture and zoom in on your camera’s LCD to judge the ruler, this isn’t the most accurate approach. The LCD resolution isn’t good enough for critical evaluation, and a trial-and-error methodology will drive you crazy. Instead, take a full series of shots at regular intervals starting at one end of the adjustment range until finishing at the other end—your shot sequence will go -20, -15, -10, -5, 0, +5, +10, +15, +20. I wouldn’t suggest using smaller increments on your first test. The adjustment steps are extremely small and you can always fine-tune further, if necessary. To eliminate variability of your camera’s autofocus system, shoot each setting three times, re-racking the focus manually on the lens between each shot. So I could identify the setting used for each sequence, I implemented the high-tech Post-it® method and made sure my note was visible in each shot. In the near future, the LensAlign PRO will ship an add-on called the Enumerator, which allows you to indicate settings in a more professional fashion using sliders on a scale.
Once you shoot a series of shots for every lens you have, use your favorite image browser to view and compare the results on your computer monitor. I use Capture One v4.7, which allows me to compare multiple shots at a 100% view. Make a note of which settings work for each lens and dial them in on your camera. If you’re having a hard time deciding which setting is really the best, you may want to increase the contrast, bring the white point in on Levels or use an Emboss filter. My approach was to bring my white point in from 255 to 200 and apply the setting to all images. I found this technique to be very helpful.
To put the LensAlign through its paces, I tested two camera kits.
The first kit was a Nikon D3 and D300 with a few top-quality lenses, including the AF-S 24-70mm ƒ/2.8 ED-IF, AF-S 70-200mm ƒ/2.8 ED-IF VR, AF-S 105mm Micro-Nikkor ƒ/2.8 VR, AF 85mm ƒ/1.4 and AF 50mm ƒ/1.4. I also tried the manual-focus Zeiss Makro-Planar T* 50mm ƒ/2 ZF to see how the same technique would work with the camera’s focus confirmation feature. The results were interesting. Certainly, all of these lenses are good and were within what I’d assume are the manufacturer’s production tolerances. But every single lens needed some level of fine-tuning except for the manual-focus Zeiss. Most of the lenses exhibited slight front focus, while the 70-200mm ƒ/2.8 showed a fair amount of back focus. All were within the correctable range.
My second kit was a Canon EOS-1DS Mark III and EOS-1D Mark III using some of Canon’s best glass. I again had some fascinating results. Some lenses like the EF 50mm ƒ/1.2L USM needed no adjustment, but others like the EF 135mm ƒ/2L USM benefited from some tweaking. I also realized that there are limits to this technique. The EF 70-200mm ƒ/2.8L IS USM couldn’t be 100% adjusted using LensAlign and AF Fine Tuning. I tested this lens at 100mm and 200mm settings, but came up with varying results. At 200mm, the lens was perfect out of the box, but at the shorter setting of 100mm, the best result was with a correction of +15. With zooms, you’ll need to evaluate your needs and do your best to figure out a happy medium. The other challenge was the EF 100mm ƒ/2.8 USM Macro. At the recommended measurement distance of eight feet, the lens alternately front-focused, back-focused, then hit dead center, all in the course of three consecutive shots. Realizing that this lens is intended for close-up macro photography, I moved to within four feet of the LensAlign. Here, the results were perfect, repeatable and required no tuning at all.
There are limits to in-camera adjustments. AF Fine Tuning is a linear correction; lens performance is nonlinear. This made optimizing the 70-200mm zoom difficult. Manufacturers do have some ability to correct for nonlinear alignment, and the LensAlign may help you identify problems. Another variable to be aware of is a phenomenon known as a spurious autofocus event—the camera changes focus between shots even though the subject has remained stationary. I encountered more than one spurious AF event in the course of my testing, which is why you should shoot at least three exposures at the same setting.
Overall, the LensAlign was an easy-to-use, effective tool for solving the problem of lens-focus alignment when combined with a D-SLR’s AF Fine Tuning feature. At $139.95, the PRO model is worth the money, even if you just want to satisfy your curiosity as to whether your lenses are working at their peak performance. And if you’ve been struggling with achieving sharp focus, this may be the answer you’ve been looking for.
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