All of today's cameras use some form of autofocus, where the camera automatically detects the distance from the camera to the subject and sets the focus on the fly. With each generation of new cameras, engineers continue to advance the intelligence and capability of digital autofocus, transforming it from a clumsy yet interesting concept into a functional piece of photographic technology. Having said that, although the technology has improved greatly, it is by no means automatic or foolproof. It's important to understand a bit about how the focusing system in your camera works, so that you can get the best results and avoid focusing mistakes. The primary types of autofocus are known as active and passive systems. Active autofocus sends out an infrared signal that is reflected by the subject, allowing the system to calculate the distance to the subject and set the focus. Passive autofocus systems determine the subject's distance from the camera based on the light and contrast in the subject matter. Passive autofocus systems are less desirable in that they are limited by the available light; they have trouble focusing if there is little or no contrast in the subject matter or if the scene is too dark. Active autofocus works well in most instances, although its infrared signal can prohibit you from focusing through glass windows or other transparent objects. Autofocus cameras define active areas within the viewfinder that act as focusing zones. When one of these areas is active, it reacts to areas of contrast and edge detail, reflecting back information to both passive and active systems for the reading. The number, placement, and sophistication of these focusing zones vary based on manufacturer and price point of your camera, but it's safe to say that all autofocus implementations are a marked advancement from where we were just a few years ago. Montalcino Ring Olivetto Trees Duomo Statue
Controlling AutofocusThe skill you need with the auto focus technique is being able to quickly identify the desired focus point in your image and select the nearest focusing zone in the viewfinder. If the subject is not aligned with the focus zone of the viewfinder, the camera will focus on the background or the foreground, and the subject will be out of focus. If your camera is older or more of a point-and-shoot model, you might not be able to select specific focusing zones, although your owner's manual should outline where the focusing zone is and how to optimize focus. In many instances, you can place the subject in the center of your viewfinder, hold the release button halfway to focus, and then recompose your image before pressing the release button the rest of the way down to take the picture. This approach to focusing is similar to a feature called Focus Lock, which is available on many advanced cameras. Instead of pressing the shutter halfway, Focus Lock allows you to center the subject and press the Focus Lock button to set the focus, then recompose the scene as desired. Digital SLRs and advanced all-in-one digital cameras allow you to select which focusing zone is active as you're shooting. Some Canon models use eye-tracking technology that allows you to simply look at a zone in the viewfinder in order to activate it (the camera tracks your eye movements to determine the focus zone you want to use). Nikon has a button that toggles through all the available zones. Most systems work well with a little practice, provided that you understand what it's doing. Duomo Scaffold III (detail) Duomo Scaffold III When to Switch to ManualAlthough autofocus can be a life saver in certain situations when you need to react quickly to get the shot, you can also see that it has its limitations. As a result, be mindful that you can switch your SLR camera from autofocus to manual mode to set the focus exactly where you want it by adjusting the focus ring on the lens barrel. (If your camera doesn't offer a manual focus feature, it's a good reason to consider upgrading to a more advanced model.)
Low LightLow light is probably the most common reason for switching to manual focus. Passive systems require enough light to bounce off your subject and reflect back into the system, and if they don't confirm a focus point, they usually don't allow you to shoot the image. In these circumstances, the only way to shoot an image may be to switch to manual mode and do your best to focus on your own. Although active systems create their own beam of light for focusing, the beam can misfire and hit the wrong area of the image. Although the active autofocus camera will let you shoot, the results from the misfired autofocus beam might not be visible in the dim viewfinder. Thus, with both active and passive autofocus systems, it's a good idea to take your time in low-light situations and switch to manual as necessary to get the shot right. No EdgesAutofocus systems look for contrasting edges to determine where the subject is in relation to the camera. If you're shooting a soft or muted subject, such as a foggy meadow or diffuse sky, the camera might not be able to find enough edge contrast to set the focus. In these cases, you should switch to manual and set the focus yourself. Some autofocus systems detect only horizontal or vertical edges and might have a problem if the appropriate edges are not visible to the focus sensor because of a lack of contrast. Problems with edge orientation can be avoided by turning the camera on its side while focusing. In either event, you can focus on another subject at the same distance and use the Focus Lock feature or switch to manual focus. Montepulciano Altar III Glass and TransparencyAs mentioned previously, glass presents a problem for both passive and active focusing systems because both systems tend to get fooled by the reflections off the glass. There's not much middle ground on this one: Switch to manual whenever you're shooting through glass and set the focus as needed. |