Tracking Down Autofocus Issues (Part 2)

As I've written before, there are no "magic settings." Autofocus is probably the most "unmagic" of them all, as how you set the focus options is highly conditional to the situation you're in and the subject you're shooting. I can't emphasize that enough, and I don't think Nikon has paid enough attention to that fact, either (some of the things I might want to change on the fly without taking my eye from the viewfinder or my hands off shooting position can't be done; it's one of the reasons why most pros switch focus to the AF-ON button, as that at least gives us the opportunity to control when the current focus settings are being used).

That said, there are some common things that get lots of people in trouble. Let's look at a few of them:

  • Don't steal light. You've slapped a TC-20e and a polarizer onto your 70-200mm f/2.8. While that lens works just fine without either of those two things, with both of them added you're at or beyond the level of light required by the autofocus system (except for D4 and D800, and even then you're close). Technically, Nikon says f/5.6 is the limit (f/8 on D4 and D800), but it's a bit beyond that. But a zoom lens is not t/5.6 with a TC-20e, it's more like t/6.3, and adding a polarizer certainly puts the light reaching the autofocus sensors at more like f/8 levels. Do yourself a favor: master focus on your system with no TCs and no filters. Then be careful as you add those things back to watch for any degradation in focus performance. There's a limit to how much light you can rob from the system and still expect it to perform at the highest levels.
  • Acquire focus first. If you start with your subject out of focus and hope that the focus system will dial it in after you mash the shutter at 8 fps, you're going to probably be disappointed. The D3/D4 bodies are better than this than the D7000, D300 and D700, but you still have to worry about viewfinder blackout time. If the viewfinder is blocked by the mirror being up, so is the autofocus system. Thus at 8 fps the system is only getting snippets of focus information intermittently. That's usually enough if the system had acquired and started tracking focus prior to your mashing the shutter release. But if the system had no tracking info prior to the sequence, it'll struggle to catch up. That's especially true of the D7000, D300 and D700, with their slightly longer blackout times. And couple this with stealing light (above), and you're in real trouble following motion.
  • Be a D3x. One comment sent to me was the assertion that the D3x focuses better than the D3 or D3s. Not that I know of. At 1/1000 and 5 fps the D3x has more mirror-down time than the D3 and D3s do at 8 fps (see above). Guess what that does for tracking accuracy? So, stop the insanity of creating hundreds of out of focus shots each session and set 5 fps instead of 8 fps. Doh! 
  • Fill the frame. A problem I see a lot, especially with birds in flight, is that the shooter just didn't use enough lens. Those autofocus sensors are pesky things. They're not quite the shape and size the viewfinder indicates, and if the one being used for focus is more off the thing you want focused than on, your focus probably won't be where you want it to be. When you compound that with having the subject covering an eighth of the frame or less, you're in real trouble with the autofocus system. It simply may not be focusing on the thing you want focused. Ironically, I find this to be slightly more of a problem with the DX cameras than the FX ones, despite the fact that the DX sensors cover more of the frame. It's precisely that fact that's the problem: with small subjects the sensor is more likely to see something you didn't want it to on the DX bodies. 
  • Failure to anticipate. I've written this before, but I'll repeat it here: if the bird I'm shooting is headed left, there's a good chance that I'm using a left-hand focus sensor. Why? Because I don't want the focus system to pick up the bird's body—I want focus on the eye. At certain flight angles, the eye will be in the same plane as the middle of the body, but at many flight angles it won't be. So I try to anticipate where the bird is going and put my sensor on where the head is likely to be. This is easy enough to do in a place like Bosque, where the birds tend to take off and fly in very predictable directions. Note that the natural inclination is to center the bird, but when you couple centering with the previous problem (not filling the frame), you're asking for trouble.
  • Not steady enough. Every amateur wants to handhold a 70-200mm lens with a TC-20e. The pro next to you has his 400mm on a tripod with a Wimberley head. Starting to see how these differences start to stack up together? The Wimberley, used correctly, should allow you to pan smoothly with the motion and keep the subject aligned to the autofocus sensors. Most amateurs I've watched handholding are actually wandering back and forth across their subject. The telltale sign? You took an 8 fps sequence of a moving subject and the subject isn't in the same place in each frame. There's a tendency to overshoot corrections while handholding if you've got poor handholding (or Wimberley) technique, so the subject wanders back and forth in the frame, causing the autofocus system to work overtime trying to figure out what's going on. The D3 models, D4, D300, D300s, D700, D7000, and D4 all have color recognition integrated with the focus system, which helps, but I've found that this works best with human skin tones, not animal tones (hint to Nikon engineers: if the outer areas of the frame are blue, the non-blue is likely the subject!). And it works better on the pro FX bodies than the DX bodies, likely because the "horsepower" of the pro FX body CPU is higher. Bottom line: if you're going to shoot bursts of shots, you need to keep the subject framed as consistently as possible.
  • Not enough practice. I'm not really known as a birds in flight shooter. But I go on bird shoots once or twice a year because I find them to be very good autofocus practice. I'll find that the first hour or two of shooting birds in flight my keeper rate is lower than it will be a few hours downstream. After a few days of such shooting, I'm usually getting exactly what I expect from the focus system. All of that improvement is due to me practicing and responding to what the focus system can and can't do. So, let's say you've got an expensive trip to shoot eagles in Alaska coming up. If you didn't practice with your local flock of seagulls prior to that trip, you're doing yourself a disservice.
  • Lock On versus Off. This is one of the trickier aspects of Nikon's focus system, and even I have to test my assumptions from time to time, as other things interact with this setting. Basically, Lock-on is part of Nikon's attempt to deal with the "not steady enough" problem I note above. (It didn't start out that way; I believe the origins of Lock-on were to try to deal with moving foreground distractions, like blowing tree limbs and leaves.) Here's the way to think about this setting: if the system thinks that it has lost the subject it was focusing on (perhaps because you overpanned), how long will it wait for that subject to get back on the sensor before starting to hunt for something fore or aft to focus on? Sometimes you want the system to go elsewhere (for example, you shifted your framing from one football player to another). Sometimes you don't (you got off the framing you wanted for the bird for a moment but will get it back). I wish there were a hard and fast rule here, but there isn't. What you have to do is watch for what the focus system is doing. If it's jumping from a foreground subject to a background too often, you may have too short a Lock-on setting. If focus is not jumping between different subjects as you want it to, you may have too long a setting. But this gets back to the previous item: practice.

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