Many years ago, when SLR autofocus systems were coming into vogue, many decried autofocus as less accurate than manual focusing. Magazines such as Popular Photography (before being body-snatched by Modern) ran studies showing that using AF reduced resolution from 100lp/mm to 50lp/mm. There was mass hysteria. Slowly, though, as the years passed, we became more accustomed to our CCD'd overlords. Today virtually every SLR sold is autofocus; SLR focusing screens are largely vestigial, and although speed has improved and tracking systems created, actual AF accuracy has not improved one bit. This is a brief list of practical tips to help you increase the usefulness of the autofocus system on your camera.
1. Stop blaming inanimate objects. In a very small number of cases, there are equipment malfunctions. In all others, it is human error, whether in judgment or failure to understand how focusing systems work. All focusing systems are susceptible to the latter, whether they be rangefinders, focusing screens, infrared, or the passive AF discussed here.
2. Find the contrast. The passive AF systems in a compact cameras is looking to maximize contrast. The more out of focus a scene is, the less contrast it has. This type of passive AF system uses CCD elements to detect the contrast level of the scene as transmitted by the lens. On AF SLRs, the system is phase detection. This system gauges focus by splitting the image the lens sees into two smaller images and then uses a line sensor to determine the distance between the two sensors. Whether these images are closer together or farther apart than a reference tells the camera whether it is focused ahead of the subject or behind (respectively), and by how much. The camera then computes how much it has to move the lens and does it.
On either system, if a scene (or more accurately, the part of it seen the AF sensor) is a uniform color or lacks any detail, passive AF is useless.
If you can't focus it with a traditional SLR focusing aid (split prism, microprism, etc), you can't do it with autofocus. The only cheat is to use a flash that projects focusing lines on the subject.
Tip: Many cameras have sensors that are designed to detect vertical lines - this covers all early AF SLRs and the non-center sensors of many current models (D70, D200, etc.). If you cannot get these sensors to lock, try rotating the camera 90 degrees, focus and lock, rotate back, and shoot. Sometimes 45 degrees will do it.
3. Focus on something parallel to, and bigger than, the AF marking in the viewfinder. In pro bodies, AF sensors are often bigger than the viewfinder marks suggest. This is not a big deal if you are shooting a flat subject parallel to the image plane. But if you are shooting an object that is at an oblique angle, be aware that the AF system will generally focus on the closest thing the AF sensor can see. This can lead to focusing in front of where you think it should be.
Tip: If you are shooting oblique subjects, try to stop down to about f/5.6. With wide, normal and short telephotos (<150mm), this should obviate most problems.
Extra tip: If your AF sensor is pointed at an oblique object with the left side closer than the right, try acquiring focus on a point slightly to the right and then reframe. Reverse these directions if the right side if closer.
In some cases AF marks are bigger than the actual sensors (c.f. Canon 350D, D70, D100, Fuji S1, S2 and S3, and Kodak 14n). CAM900 sensors in particular have sensitivity patterns that look like lines passing through the indicated areas - so narrower in one dimension and much longer in the other. This leads to similar problems and you will want to experiment to see just where the sensor is in relation to the AF marks.
4. Close the loop. Autofocus systems are generally open loop in (S)ingle shot mode. This means that the camera looks at the focusing rate of the lens, computes how much movement is needed to maximize contrast, and then moves the lens there. Lenses with very short focusing throws can overshoot (or undershoot) the subject. It is often beneficial to switch the focus to (C)ontinuous, so the camera focuses, checks, focuses again, etc. This changes the operation to closed-loop. See what the camera is doing in terms of checking focus. When it stops adjusting, take the shot.
5. Randomize it. Refocus between shots (far-to-near or vice-versa). This minimizes the possibility of the camera picking a point that is "off" and your shooting all of your pictures at that focus point. In other words, don't put all your eggs in one basket.
6. Set for focus priority. On many cameras, especially Nikon, you have the option to maintain the focus as long as the shutter button is held down ("shutter-priority") or to refocus after each shot ("focus-priority"). If you have a choice, choose the latter. This will prevent you from shooting an entire sequence at one incorrect focus setting (see "Randomize it," above.
7. Keep your distance. One on-line reviewer constantly talks about autofocus errors in lenses. There is nothing such in screw-drive lenses. There is, however, a perfect storm that builds up as you get close to the subject. A 200mm lens at f/4 has 1cm depth of field at 1.5m. That is not even enough depth of field to pivot the camera for an off-center subject. At that distance, you are working at less margin for error than the tolerance of the AF system itself (typically 1cm at 1m with a 50mm lens). If you are lucky, you will get a sharp picture. If, on the other hand, your sensor picks the wrong part of the subject or if you move even slightly as you expose the picture (see below), you will get unsharp pictures. With manual focus telephotos, the close focusing distance was typically 1.5m and up due to limitations of making a focusing helicoid that could get long enough - and the huge lens aberrations close up. With AF lenses, which don't have helicoid limitations, the issue is more whether the AF system can function effectively at close distances.
8. Use Group-Dynamic AF and Dyanamic Area AF with closest-subject priority. I'm pretty sure Canon has an equivalent. Remember that 2/3 of your depth of field is behind the subject. Setting your camera to Group Dynamic AF with Closest Subject Priority, and the camera will look at a selection of sensors and pick the sensor aimed at the closest part of the subject. This will probably sacrifice 1/3 of your depth of field right off the bat but will assure you that you will get the 2/3 that lies behind. Likewise, Dynamic Area AF with CSP looks at the whole frame and of the sensors, picks the closest ones. This has the same risk/benefit but in fast moving situations cuts down the fumbling with AF area selection.
9. Follow through! Don't jerk the camera when you take the shot. You may displace the camera backward ever so slightly - or you may cause the camera to think that the subject is moving again and that it needs to refocus. Gently squeeze the shutter release until the camera fires. It's a lot like shooting a target rifle. If you are in the habit of quickly pulling the camera away after each shot to see the last exposure on an LCD screen, you will run into big problems - because you will start jerking the camera away as you are pressing the shutter release. A good way to see what you are doing with camera stability is to use the redeye reduction feature of your SLR flash. This will inject a delay between pressing the release and the exposure; if you do this and find your pictures come out sharper, it is because you were jerking the camera before.
10. Spend the money?! I normally don't advise anyone to spend any more than they absolutely need to; however, this is an exception. Certain people are of the opinion that every camera is built equally. While this may be true in a lot of respects, it is not true when it comes to viewfinder systems. Almost without fail, expensive cameras have better viewfinders and focusing systems than cheap ones. This was true with rangefinders: compare a Leica M3 to anything else to see the difference. It was equally true with manual focus SLRs: compare a Canon F-1 or Nikon F series to cheaper, consumer-oriented models. The rule there was that pro cameras could focus anywhere in the viewfinder; cheaper ones could only accurately focus in the center. It is more true with regard to AF cameras, where you typically get:
More sensors over a wider area - preventing problems when you focus and recompose for off-center subjects. Having sensors over a wider area requires much more precise mirror alignment, which leads to more labor, which leads to a higher price;
More cross-type sensors - allowing focus acquisition as long as the subject has some texture. This requires three-dimensional alignment of the sensors (distance, pitch, yaw), again leading to higher cost;
Faster AF processing times - getting the process rolling;
More power in the system - in Nikon systems, the power that can be delivered to a screw-drive lens is directly proportional to battery voltage. This is part of the reason why the F100 (6v) was never quite as fast as the F5 (12v) and why the D200 (7.4v) will not be as fast as a D2x (11.1v). My suspicion is that battery peak power also plays a role in AF-s and USM performance, too, since those motors are powered by the camera body; and
Shorter mirror blackout leading to better focus tracking - since AF systems can only read when the mirror is down.
In the end, don't take someone's word for it that X camera has 80% of Y camera's capabilities at 1/2 the price (reminiscent of D200 vs. D2x arguments). If the remaining 20% is, for example, the capability to do wide-aperture, closeup, off-center portraits, and that is what you do, then it is a false economy. Make sure that the focusing system can accommodate your needs.
Conclusion. Hopefully, this will give you some ideas on how to increase your AF "hit rate." If you think of any other tips, drop me a line in the guestbook.