Cameras come with lots and lots of bells and whistles these days, and some of them seem a lot more useful than other. Built-in selective color mode? No thank you. But every once in a while there’s a new feature that changes the game in a big way. Autofocus. Auto-exposure modes. Vibration reduction. To my mind, autofocus fine-tuning is one of these. What is this? Nikon calls it “AF Fine Tune,” and Canon calls it “Focus Micro-Adjust,” but basically it’s simple, and brilliant: It allows your camera to make sure your autofocus actually works up to its full potential.
Lens design is an incredibly complicated process, and like anything in life we can’t aim for perfection, just good enough. The problem is compounded because different cameras, even of the same model, respond to lenses slightly differently, and the lens-maker doesn’t have access to your camera when calibrating in the factory. Generally, most good companies produce results that are, to use a scientific term, good enough for Rock ‘n’ Roll. If you’re taking photos at f/8, it’s highly unlikely you’d ever notice a problem. But especially these days where it’s more and more fashionable to shoot with the razor-thin DoF of f/1.8 or wider, little problems become glaring. If a lens if off by even a couple millimeters, that can be the difference between an eye and an eyelash in focus. An inch? Then it’s the nose in focus, and you’re noticeably off.
In the old days, you were pretty stuck. All you could do is send the lens back to the factory and hope for the best, or actually travel to a lens technician and have them re-calibrate it to your camera. Message boards are filled with horror stories of people who sent a lens back five or six times and still couldn’t get it working right.
But a few years ago, dSLR makers figured out how to change the variances a little bit in-camera. Is the nose in-focus instead of the eye? You can tell the camera to move back an inch with that particular lens. Is the focus going back to the ear? Tell the lens to focus forward a bit. Suddenly, that slightly annoying fast lens you couldn’t quite get right becomes a useful tool. Batches of lenses that had wide reports of focus quality control-issues, like some of the wide, fast Sigma lenses such as the 24mm f/1.8, work just fine, thank you very much.
Perhaps because these work best with fast, professional lenses, and because it’s a bit tricky to get working properly, most camera-makers include this feature at the “advanced amateur” level and beyond. For instance, Nikon has it on the D300s but not the D90 (though they did include it on the D7000), and Canon has it on the 7D but not the D60. It’s such an incredibly useful feature that I wouldn’t be surprised to see it buried in menus with a “WARNING: Only touch if you know what you’re doing!” on even the most basic DSLR models in the future.
The basic working of it is extremely similar across camera lines. Go into the camera function menu with your problematic lens mounted, and you’ll see a chart with pluses and minuses radiating out from zero. These allow you to correct for front- or back-focus to varying degrees. Because modern lenses have CPU chips in them, the camera will even be able to tell two lenses of the same model apart. This is crucial because, to paraphrase Tolstoy: “All perfect lenses are alike; each imperfect lens is imperfect in its own way.”
But the catch is that there’s no automatic way to do this. If there was, the camera could just fix the problem without a chart. Basically you need to shoot, figure out the amount of front-focus or back-focus. And then shoot again and see if your calculations were correct. And most importantly, you want to absolutely make sure that none of the focus problems are due to user error, or just the tricky act of hitting a precise target at f/1.4
If this sounds like a complicated, frustrating process, well … it can be, especially if you have a lot of lenses … even more so if you have to test all of them with multiple cameras. And so a few companies have stepped in with products to make the process easier. I tested both the LensAlign system and Spyder LensCal with a variety of lenses on my Nikon D3s bodies, as well as the Sigma 30mm f/1.4 on the Canon 7D, since a number of people have reported back-focus on this lens with Canon bodies.
Both of these products are trying to perform a very simple task in an effective way, which is to be a good autofocus target and and effective measuring system that will let you see exactly where the plane of focus falls. There are only so many possible configurations for this, and so you can see that when set up they look very similar (LensAlign on left; LensCal on right):
To eliminate user error, when setting up for use you should use a tripod exactly level with the target so your lens is at a perfect perpendicular. Now, a good lens should be distance-calibrated, meaning that if it focuses accurately at five feet it should focus accurately at 10 feet, but that’s not always the case so I tended to set the lens at the distance I most often shoot with it (closer for the 24mm f/1.4, farther for the 85mm f/1.4, and so on). This also made sure that the lens had a big enough target to focus on, since hitting these from 10 feet away with a 24mm lens won’t be as accurate as you want it to be.
As you can see, in use these are extremely similar. The LensAlign chart has a lot more to it, which can make it easier to set up a perfectly accurate test. In particular the second bullseye on the right can help you make sure you’re exactly perpendicular, as you’ll want both targets to be tack sharp. But the bells and whistles come at a price. The Spyder is extremely simple to set up. It starts like this:
You unsnap it and voila! Done. In contrast, the LensAlign comes like this:
And here are the directions to put it together:
If you are someone who gets thrown into a murderous rage by Ikea furniture, you might prefer the Spyder.
And of course there is the third option of doing it on your own — which at least is better than nothing. One decent trick is to put a coin on a towel or shag rug and focus on that — the threads of the towel will really show exactly where the focus plane is falling — and in all cases remember that it gets much more accurate as you zoom in on the picture, thanks to the circle of confusion.
If you have just a kit lens, you probably don’t have to worry. If you have one camera and a 50mm f/1.8, you can probably muddle through yourself. But for me, calibrating almost 20 lenses on two D3s’s and a D3, these were invaluable tools. I’ve had lenses rescued from the scrap heap because of focus micro-adjust. If you like shallow depth-of-field or think that you will in the future, this is a feature to watch out for as you buy a camera. It’s not a bad reason to pick up a D7000 over a D90, for example.
Virtually every one of my lenses was improved by this, though in many of the cases it wouldn’t be noticeable in normal usage.