The three types of focusing systems common in modern 35mm cameras are fixed focus, manual focus, and automatic focus. Fixed-focus lenses, common to the simplest compact cameras, are preset to provide sharp focus over a given distance range (typically from a minimum of about 4 feet to infinity). Fixed-focus lenses are fine for taking snapshots of family events or vacation scenics, but they do limit your flexibility and image control. The ability to focus your camera selectively allows you to concentrate attention on a particular area of a scene.

Many SLR cameras use manual focusing. To focus a manual SLR camera, you look through the viewfinder while turning the focusing collar on the lens until the area that you want to be sharp comes into focus. The viewfinder in an SLR usually contains a split-image rangefinder, a microprism, and a ground glass to help you in judging sharpest focus (see illustration). A split-image rangefinder does what it says-it splits the image. With split-image rangefinders, you look through the viewfinder at your subject and turn the focusing ring on your camera lens until the split images in the viewfinder line up. The subject is then in focus. Split-image finders work best when there is a prominent vertical line (a telephone pole, for example) in your composition to use as a focusing guide. One disadvantage of the split-image rangefinder is that it sometimes blacks out when you use it with telephoto lenses, obliterating part of the image area. If your camera has a microprism area (see illustration) or a ground glass for focusing, you just turn the focusing ring until the subject looks sharp in the viewfinder. The microprism is a ring in the center of the viewfinder that exaggerates the unsharpness of out-of-focus subjects and therefore makes correct focusing easier to judge. 

Each focusing aid works best in certain situations. For instance, you may prefer split-image focusing when you're photographing a building because there are straight lines to focus on. But if you're taking a picture of a crowd of people where prominent lines are hard to find, the microprism and ground-glass systems of focusing are easier and faster. In addition, the ground glass offers the advantage of showing you what part of the picture is in the plane of sharpest focus relative to other parts of the picture. The ground glass also lets you preview depth of field over the entire image area if your camera has the feature that permits this. 

Practice focusing your camera until you do it so naturally that you don't have to give it a second thought. 

Electrical contacts in the lens and camera enable the computer in an autofocus SLR to set the autofocus and the aperture, and adjust shutter speed or aperture according to the lens focal length. 

Although the engineering behind an auto-focus camera is complex, using one is easy. In both compact and SLR cameras, the viewfinder has a center target to show what the camera will automatically focus on. To get sharp pictures, aim your camera to superimpose this target on the main subject. 

The obvious problem is that you won't always want to put your main subject in the center of the frame. To circumvent this, most auto-focus cameras have a focus lock. With it, you focus on your subject in the center of the viewfinder and then lock the focus. Once you lock the focus, you can recompose the scene. 

With most auto-focus cameras, partially depressing the shutter button activates the focus. As long as you do not release the button (or take a picture) focus will remain locked on your chosen subject. Some cameras have a separate button that locks the focus until a picture has been taken. For the focus-lock technique to work, however, the camera-to-subject-distance cannot change. If the distance does change, the subject will likely be out of focus. Refocusing is a simple matter of re-centering your subject and again partially pressing the shutter release button. 

More sophisticated SLR auto-focus cameras may offer a choice of two autofocus modes: single shot and servo. The single-shot mode is better for photographing stationary subjects. The camera will not fire until the camera has confirmed sharp focus. The servo mode works well for photographing moving subjects-a girl on  bicycle, for instance. It will continue to focus until the instant of exposure. One drawback of the servo mode is that many cameras will fire even if the subject is not in sharp focus (if the subject has continued to move as the shutter is tripped, for example). Some very advanced SLR cameras are actually able to overcome even this problem by measuring the speed of a moving subject. The camera calculates how far the subject will move during the split second it takes to make an exposure and adjusts the focus accordingly. Consult your manual for advice on photographing moving subjects. 

Some situations and subjects can cause focusing errors or failure. For instance, the camera cannot automatically focus on subjects behind other objects, such as a fence or a branch. If you are using a leafy tree limb to frame a barn, you must be certain that the focus area in the viewfinder is not seeing the limb or it will cause the lens to focus on the limb and make your main subject, the barn, out of focus. 

Because most auto-focus systems work by comparing the tonal contrasts within a scene, they may balk in both low-light and low-contrast situations such as a foggy scene. 

Most cameras will beep or flash an indicator to indicate a focus problem. In dimly lit situations, the camera may tell you to use an accessory flash. Some flash units have infrared emitters that enable the camera to focus automatically in almost total darkness, and the flash provides enough light for picture taking. Brightly backlighted scenes also trouble most SLR auto-focus systems because the contrast in the scene is intense and changes rapidly if you move the camera even slightly. 

In SLR auto-focus cameras, the sensors are behind the lens. As long as you have an unobstructed view of your subject, the camera will focus. In compact auto-focus cameras, however, the auto focus sensors are behind windows on the front of the camera. It's very important not to block these windows with a finger or camera strap, or focusing will be impaired. 

Finally, some auto-focus SLR cameras have trouble focusing on subjects that contain predominantly horizontal lines-clapboard shingles on a house, for example. You can get around this difficulty by turning the camera on a slight angle, locking in the focus, and then recomposing the scene.

Most SLR auto-focus cameras have an override that allows you to focus the lens manually. This can be an important feature if, for example, you want to manipulate focus for a special effect or to focus on a subject in a difficult situation-such as photographing a zoo animal through the bars of a cage. Many of these cameras have a feature called "focus assist" that will provide an audible or visible signal to tell when the subject is in focus.

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