A full 27 years have passed since the first commercially successful autofocus SLR was introduced (the Maxxum 7000), and the technology has been significantly improved since then. Even so, some photographers consider AF suitable only for snapshooting. Others insist that even the best systems are far from reliable. They relate stories of missed opportunities while the AF system hunted for focus in low light or set focus for an inappropriate subject area.
The default setting for autofocus may not always set focus on the most important subject, such as the subject's face in this wide angle composition. Whenever you encounter a problem with autofocus, simply switch to one of the tactics discussed in a moment. Photo: Peter K. Burian
The above may suggest that autofocus is unreliable in general, but that’s far from accurate. Having tested numerous interchangeable-lens cameras, I know that the current technology is incredibly successful. If you’re using an old camera with an outdated AF system, you may not consider autofocus to be as successful. However, current interchangeable-lens cameras often provide fast, reliable focus acquisition thanks to new technology and features.
Even so, there will be occasions where the AF system focuses on the wrong subject or is very slow to respond. Whenever you encounter a focusing problem, consider the following techniques.
Don’t expect AF to read your mind: Even the most “intelligent” system cannot determine your photographic intentions. Its automatic focus detection system is designed to set focus quickly on a "reliable" target. In a close-up portrait for example, an AF system will often select the nose. In an ultra-wide composition, with subjects at various distances, the system may select one that's not the most important in your opinion.
Use a single AF point: Automatic focus point detection can be useful in action photography when you must shoot quickly. (When the subject is moving, also be sure to set the Continuous, AF-C, mode.) In other situations however, it’s best to manually select only a single focus detection point, the one in the center of the frame. (For static subject's use the Single Shot AF-S mode.)
Point the lens so the primary subject is centered to ensure that the camera will focus on it. But there is no need to take photos using a bull's-eye approach to composition. Simply maintain slight pressure on the shutter release button while you recompose for better framing. When you're ready, press the button all the way down to take the shot. (Note: Many cameras also have a separate focus-lock feature that you can use instead of the shutter release button.)
Use the viewfinder, not Live View: Most digital SLR cameras provide the best autofocus performance when you are not using live view. In situations where the AF system is slow to focus, turn Live View off. Instead of framing the photo on the LCD monitor, use the camera's viewfinder and you'll get the fastest/most reliable autofocus.
Minimize AF frustration in low light: Although a high-tech camera’s low-light focusing ability should be very good, even the best systems benefit from thoughtful technique. Activate only the central focus detection point since that's the one that is most sensitive. Remove any filter with dark glass, such as a polarizer. Focus on a part of the scene that has good contrast and is not overly dark. Then use the focus lock technique discussed above while recomposing for your preferred framing.
If you focused on a light-toned area such as a bright lamp, your photo may be too dark. In that case, set a +1 exposure compensation (with the +/- button and rotating the camera's dial). Take the shot again and it will be brighter. Afterwards, remember to reset exposure compensation to zero.
In this very dark theatre, the autofocus system struggled when I tried to focus on the performer's eye. The solution was simple: focus on the brighter flowers on her head, and use +2/3 compensation to prevent a dark (underexposed) image. Photo: Peter K. Burian
Find a more reliable subject: As your camera's instruction manual will indicate, autofocus systems cannot easily acquire focus on certain types of subjects. It may struggle with a flat wall or with scenes including prominent vertical bars. Don’t give up if the AF system balks.
If your primary subject makes for a poor target for the AF sensor, try focusing on something else at a similar distance: a door knob instead of a wall without texture, for example. If your camera has difficulty focusing on a certain type of pattern, turn it to diagonal angle; now it should find focus easily. With small subjects surrounded by a framing object such as foliage, use only the central AF sensor. If shooting through a window or aquarium glass, move close to the pane so the system does not try to focus on the surface.
Switch to manual focus: Although I use AF for some 90% of my photography, manual focus remains a useful alternative at times. This approach is often useful in close-up photography, for example. In portraiture, “macro” nature and other close-up work, critical focus on a small, key element is essential. Getting the point of focus just right can change an image from acceptable to technically perfect.
Practice makes perfect: Although the latest AF systems are highly effective, it’s essential to recognize situations that call for taking over some control. Not all of the techniques are intuitive, especially when you first start using a new full-featured DSLR. After studying the owner's manual, with camera in hand, go out and start shooting with each of the options. Don’t be reluctant to shoot a lot of photos until you become proficient in problem-solving with autofocus. This investment in time and effort will pay dividends; it will help to minimize frustration and to maximize your satisfaction with sharp, properly focused images.
- Peter Burian teaches a couple of excellent online photo classes - Mastering the Digital Camera and Photography and Mastering the Canon EOS Digital Rebels - at BetterPhoto's digital photography school online.
- Peter's photography also appears in a pair of books co-authored by Jim Miotke and Kerry Drager: The BetterPhoto Guide to Photographing Light and The BetterPhoto Guide to Creative Digital Photography.