When you study most stereo photos, you can see a considerable area of the scene from near to far that appears sharp. Even though theoretically only one narrow plane is critically sharp, other parts of the scene in front of and behind this plane appear acceptably sharp. This area is referred to as depth of field. In stereo photography you usually want all parts of a scene to fall within this available depth of field. The parts of the scene that do so become less and less sharp the farther they are from the plane of critical focus. Eventually they become so out of focus they no longer appear sharp at all. The nearest and farthest points that are acceptably sharp mark the planes of near and far focus. These limits are usually not visible as exactly defined boundaries. Nor can you usually find the plane of critical focus by looking at a picture. Instead, sharp areas imperceptibly merge into unsharp ones.

Normally, depth of field is not evenly divided by the plane of critical focus. At normal shooting distances, about onethird of the depth of field is in front of the plane of critical focus (toward the camera), and two-thirds is behind it (away from the camera). When the camera is focused very close to a subject, as it would be when doing close-up photography, the depth of field becomes more evenly divided.

Often it doesn't matter so much exactly what you are focused on. What does matter is whether or not the parts of the scene you want to be sharp fall within the available depth of field. When you want a large part of the scene to be sharp, as you do in most stereo photos, you can increase the depth of field. When you want less of the scene sharp you can decrease it.

To control depth of field, you have three factors to work with.

• Aperture size. The smaller the aperture, the greater the depth of field. The larger the aperture, the shallower the depth of field. The aperture often widens in bright light and if you pick a higher ISO or slower shutter speed.
• Camera-to-subject distance. As you move father from the subject you are focused on, without changing the focal length of your lens, you increase depth of field. As you move closer, you decrease it.
• Lens focal length. Using a wide-angle lens or zooming out increases depth of field. Using a long lens or zooming in decreases it.

Each of these three factors affects depth of field by itself, but even more so in combination. You can get the shallowest depth of field with a lens zoomed in on a nearby subject using a large aperture. You get the greatest depth of field when you are far from a subject, with the lens zoomed to a wide angle, and using a small aperture.

One last word. When a scene extends to the far distance, many photographers unthinkingly focus on that part of the scene. When you do so, everything from that distant point and beyond will be sharp. But since one-third of the available depth of field falls in front of the point on which you are focused and two-thirds behind it, you are wasting two-thirds of your depth of field. As a result, some other part of the scene in the foreground may not be included in the one-third remaining depth of field and consequently will not be sharp.

Instead of focusing on the most distant part of a scene, focus on a point one-third of the way between you and it. This brings forward the plane of critical focus and the plane of near focus, increasing depth of field in the foreground of your picture. The plane of far focus also moves forward, but still falls beyond the farthest point in the scene. This new point of focus is called the hyperfocal distance. You can use this procedure not just for landscapes, but whenever you want to shift depth of field toward the camera.