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Your Camera: What Is Auto Focusing?

Focusing was the last great hurdle of camera automation. Automatic film advance, automatic flash, automatic exposure - these amenities have long been a fact of point-and-shoot life. Automatic focusing - auto focus, for short - is a more recent innovation. But auto focus was worth the wait, relieving one of the great anxieties of snapshot photography. Auto focus has saved lots of pictures that otherwise would have been lost to the vexing back-and-forth of old-fashioned manual focusing.

You - and only you - must make sure that your point-and-shoot's lens is properly focused when you take a picture. Of course, however, a point-and-shoot will take a good percentage of sharp pictures if you just glance through it and snap away. Left to its own devices, your camera will focus automatically. It will focus automatically on whatever chance places in its path. But you simply can't leave focusing to chance. That's how bad pictures happen to good people.

What is Auto focusing?

So what is focusing, really? People tend to use the term pretty blithely; they at least seem to know that it has to do with making a photograph sharp. Basically, focusing is the process of moving the lens in and out until it forms the sharpest possible image of the subject on the film. Focusing is necessary because for every subject at a different distance from the camera, the lens must be at a different distance from the film to create the sharpest possible image.

An auto focus point-and-shoot uses sophisticated technology to measure the distance to the subject. It relays this information to a motor in the lens. The motor then moves the lens to the proper distance from the film.

Very Similar To Your Eyes

Though their mechanism is different, your eyes auto focus, too. They do so with such precision and speed, in fact, that you don't even have to think about it. Well, at least until you start needing glasses to find your glasses, or wishing you had arm-extenders to read.

To see how your eyes auto focus, put your hand as close to your face as you can and still see it clearly. Now look right past your hand at a distant object - something across the room from you. Keep looking at that distant object and try to see out of the corner of your eye (that is, without looking directly at it) what your hand looks like. Blurry, isn't it? Now look back at your hand. Again, out of the corner of your eye, see what the far object looks like. Now it is blurry. Your eye refocuses automatically when you look at something that's at a different distance - rather like an auto focus point-and-shoot camera!

When you look through an auto focus point-and-shoot's viewfinder window, though, you don't see that focusing process, even though it's happening inside the camera. The entire scene remains sharp regardless of where the lens actually focuses. For most people, this fact isn't bothersome. But it may be strange for the many photographers switching to a point-and-shoot from an old 35mm SLR (single-lens reflex). An SLR camera's viewfinder allows you to view and compose the subject - and with manual-focus models, to focus it by hand - through the lens itself. You can see one part of the scene become sharp and another become blurry as the lens refocuses, just as you did with your hand and that distant object. But former SLR photographers just have to trust that their point-and-shoot camera is doing its auto focus thing!

Auto focus VS. Fixed-Focus

Auto focus is not a necessity for good pictures. Tons of less expensive (under $50) point-and shoots, and all one-time-use cameras, rely on what is often called focus-free operation. This term should really be fixed focus, because it means that the lens's focus was permanently set at the factory. This fixed setting gives you pretty sharp pictures if your subject is at an average snapshot distance (eight feet or so), and less-sharp-but-acceptable pictures if your subject is a little closer or farther away. But if you get too close to something, it will bit appear sharp in your print. With fixed-focus, focus-free cameras, four feet is about as close as you can get and still take reasonably sharp snapshots.

The best way to tell whether you have an auto focus or a fixed-focus point-and-shoot is simply by looking through the viewfinder. If you have an auto focus model, you'll see a small set of marks - often in the form of brackets or a cross shape - in the center of the frame. These marks, called the focus point, tell you where the camera is focusing.

If you have a fixed-focus point-and-shoot, its viewfinder lacks the focus point. A fixed-focus camera also lacks the green focus-OK lamp found on auto focus models. The lamp is built into the side of the view-finder eyepiece or is just inside the eyepiece frame; occasionally it's on the bottom of the eyepiece. Wherever it is, you can see it out of the corner of your eye when you look through the viewfinder.

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