By Ellis Vener
Reliable autofocus is the result of a technological dance between two complex systems: the lens and the camera. Modern high-end DSLR cameras that have active live view come with a second autofocus system based on contrast detection. To avoid overcomplicating things I'll just set contrast-detection autofocus aside to discuss another time.
So how does standard autofocus, the kind you use when viewing your scene through the viewfinder, work? The big reflex mirror — the one that directs the image-forming light up through the ground glass, pentaprism, and eventually out through the viewfinder has multiple semi-transparent spots in it and some of the light passes through these spots to a secondary mirror that directs the light downward to an array of autofocus sensors in the bottom of the mirror box. The light rays are now directed to a pair of receptors and the computers behind the receptors turn the light into electronic signals and analyze the signal for differences that signify an edge or a tonal differences. As the late Bruce Fraser liked to say, "Difference is detail." The image is in focus when the peaks and valleys in the two signals coincide and come into phase with each other. This is why this is called phase detection autofocus. All this happened in milliseconds.
When it works, and it mostly does, phase detection autofocus works really well. But the individual lens and individual camera body are two complex systems working together. If you want really sharp photos and are not willing to settle for photos that are slightly out of focus, the two unique components need to be calibrated so they work together perfectly, otherwise you get some degree of back focusing or front focusing. This happens because the computer in the camera and the autofocus motor in the lens are not communicating properly.
Until a few years ago this kind of lens/camera tuning could only be done by an authorized repair center. The potential for misalignment also predates both the introduction of digital imaging and autofocus. Some photographers would test multiple lenses of the same model to find one with a real sweet spot for their camera body and would then have the focusing alignment of cameras and lenses recalibrated every so often.
In 2007 both Canon and Nikon started including autofocus micro-adjustment tools into their higher end cameras so individual photographers could do their own testing and AF tuning. Starting with the EOS 5D Mark III and EOS-1DX cameras, Canon went a step further and gave users the ability to tune autofocus accuracy can at the longest and shortest focal lengths of a zoom lens.
While you can make your own target and use your naked eyes to evaluate the results, there are at least three companies which make targets and software to automate and improve the accuracy of the results. Even though I prefer the LensAlign Mark II plus FocusTune software combination from Michael Tapes Design, Datacolor SpyderLensCal (target only, no software), and Reikan FoCal software also work well.
Whichever target and software you use the process is the same: you shoot several frames of the target at different AF Micro adjust settings and repeat until you find the best setting. I start by shooting 5 frames each at -20, -15, -10, -05, 0, +05, +10, +15, and +20 and have the FocusTune software evaluate the results. I then shoot a second round of tests around the best results. If both my eye and FocusTune agree that the best results were at -05, I'll shoot a second round of five frames each at the -07 to -03 settings. Once I've nailed down an adjustment setting that looks best I go out and make real world photos.
The qualitative difference between the untuned and tuned result are usually pretty obvious. Having your focus perfectly dialed in also has a psychological benefit of boosting your confidence in the quality of your work.
As good as the adjustment might be, though, there can still be problems: the AF might work great at one range of subject-to-camera distances but not quite so great at others. To solve this problem a more complete reprogramming of the lens is necessary.
Recognizing this, Sigma has recently has gone a step further and introduced the Sigma USB Dock and Sigma Optimization Pro software. The dock and software allow you to reprogram autofocus performance and update firmware for the new Global Vision line of Art, Contemporary, and Sports lenses. The 120-300mm F2.8 DG OS HSM Sports lens (there is only one sport lens currently in the Global Vision line) allows for AF speed, focus limiter, and optical stabilization customization as well.
To tune the AF performance, shoot a target at four specified distances. With the zoom lenses you can do this for four focal length settings. My experience with Sigma Optimization Pro and the 35mm F1.4 DG HSM is that you can get lucky and need no fine tuning at one distance, but it takes a minimum of two and often three rounds per distance. The process takes roughly 40 minutes with a fixed focal length lens, so plan on about two hours for a zoom.
There are a couple things to consider. First, all of this fine-tuning takes time. Second, you must be very precise about how you attach and remove the lens from the dock. Attach the lens to the dock and then the dock to the computer. After adjusting a lens's setting, the process goes in reverse: disconnect the dock from the computer and then the lens from the dock. I discussed this with a Sigma technician and he advises users to never hot-swap lenses on a camera; turn the camera off first and then swap lenses. Turning off all power to the lens removes the possibility of bridging the contacts on a lens or on a camera body.
If you are using the lens with more than one camera, set your primary body's AF micro adjustment to zero and then fine-tune the focusing using Sigma's software. Set your backup camera bodies' AF Micro-adjustment to match your primary body's AF performance. In practice this works very well.
That's a lot of testing and you are probably wondering if it's really worth all of the time or is it just pointless hairsplitting. The argument for doing it is pretty straightforward: In return for a small investment in time and money (the dock retails for just under $60 and the software is free) you get your full money's worth of image quality out of both your lenses and camera.
I'm glad to see Sigma innovate in this manner. They seem to really be making an all out quality push these days and perhaps Canon, Nikon and Sony will release similar products to Sigma's Optimization Pro and USB Dock. It won't take care of all of the performance tuning an authorized service center can perform but it's a start.