How Autofocus Cameras Work

An auto focus (AF) camera uses a miniature computer-operated motor to focus the camera lens. The lens is moved in or out until the subject comes into the sharpest possible focus, which depends upon its distance from the camera lens. So in order to work, the auto focus system must somehow determine how far away the subject is from the lens.

If the camera is a point-and-shoot camera, it probably uses an active AF system, which means that the camera emits an infrared signal to detect the distance to the subject. The microprocessor then computes the distance using that signal in one of three ways: by measuring the intensity of the infrared pulse of light bounced off the image, by measuring the amount of time it takes to reflect from the image, or by triangulation (calculating the distance through the relationship between sides and angles).

Active systems work best when the subject is within 20 feet or so. Also, in certain situations, the infrared sensing can become confused. For example, a picture cannot be taken through a window because the infrared beam bounces off of the glass. Open flames within the frame can also confuse the sensors, and a very dark surface can absorb the infrared beam.

A passive system uses a computer analysis of the image itself to compute the distance. This type of system is found in SLR cameras with interchangeable lenses, and typically uses sensors that compute the contrast of the image. The camera sees light from the scene using a charge-coupled device (CCD).

This device consists of a strip of 100 to 200 pixels, which is hit with light from the image. The microprocessor searches for the greatest difference in intensity between lights and darks in adjacent pixels, while the camera continuously adjusts the lens, resulting in the sharpest image. If the image is very low contrast or the scene has low light, the camera will have trouble focusing.

In addition, these systems usually read vertical detail, so you should focus on a vertical edge of the subject (such as a face) or hold the camera in the vertical position for horizontal details such as mountains in the horizon. Some of the newer, more expensive models have sensors that focus on both vertical and horizontal details. In addition, some pricier cameras use both active and passive AF systems.

Prior to the introduction of autofocus cameras, photographers relied on manual focusing to get the sharpest picture, and less expensive cameras used fixed lens cameras that worked fairly well for most subjects. Both types of focusing systems had drawbacks, however, so when Canon introduced the first fully automatic AF 35mm lens-shutter camera in November 1979, it was an exciting innovation.

This camera was called the “AF-Plus Motor-Drive camera but was nicknamed the AF 35M Autoboy or SureShot in the United States. It was said to work regardless of brightness, contrast, or distance; and became very popular.

In 1985, the Minolta MAXXUM 700 was introduced, the world’s first auto-focus SLR. Other manufacturers followed, and soon many photo bugs were trading in their manual focus cameras for the new auto-focus SLR models. Others, however, took a wait-and-see approach, and many purists stayed with their tried and true manual SLR cameras, not trusting the new auto-focus systems.

The new AF camera, besides being a great time-saver and a boon for eyeglass wearers, allowed photographers to do things that were nearly impossible with manual focus cameras. For functions such as focusing on moving objects and for action series shots that don’t allow time for manual focusing, autofocus was invaluable. AF also allowed the photographer to concentrate more on composition than on the mechanics of focusing, and made candid shots of people a snap, without time wasted on focusing and risking the loss of the moment.

However, in addition to the problems with active and passive systems discussed above, there were other disadvantages to AF cameras. The subject had to be centered in the picture frame at the center focal point. Often this is not the ideal placement for the subject, so the photographer had to first compose the picture, move the camera to focus on the subject, press and hold the shutter button halfway down to lock in the focus, then return to the original composition and snap the picture.

Purists maintained that this clearly illustrated loss of creative control; that no AF camera could allow selective composition and focus as well as the person operating it. To solve the problem, later models included several outside focus points around the center that could be selected for focus. Most recently, Canon’s EOS ELAN series features a system of exclusive Eye Controlled Focus, in which the AF system tracks each glance or movement of the eye for the fastest, the easiest method of selecting a focus point. The ballot is still out on eye-controlled focus and if it truly works as well as Canon claims.

Of course, being able to switch between auto and manual focus allows the photographer maximum creative control. The trick is to know when the auto focus feature can be used effectively and when to switch to manual focus. In certain situations, auto focus can be used initially, then tweaked manually. It can be combined with auto exposure and auto flash to allow for more concentration on composition.

Auto focus can be a phenomenal tool when used appropriately and knowledgeable. Study your owner’s manual and gain a thorough understanding of how your AF camera system works. And of course, experiment! You will discover just how much auto focus can improve and enhance your photography.

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