Plane of critical focus, focus areas and techniques, depth of field

If you look around you—the book in your hand, the chair across the room, the far wall—everything seems to be sharp. That is because your eyes refocus every time you look at an object at a different distance. But the sharpness you see when you glance at a scene is not always what you get in a photograph of that scene. To understand why not, you have to understand focus and depth of field.

Focus

Focus is only one of the factors affecting the apparent sharpness of your photographs, but it is a critical one because it determines which parts of the picture will be sharpest—called the plane of critical focus. You will have much more control over the final image if you understand how focus relates to the overall sharpness of a scene.

Imagine the part of the scene on which you focus as a flat plane (much like a pane of glass) superimposed from one side to the other of a scene, so that the plane is parallel to the back of the camera or the image sensor. Objects falling exactly on this imaginary plane will be in critical focus, the sharpest part of your picture. This plane of critical focus is a very shallow band and includes only those parts of the scene located at identical distances from the camera. As you point an autofocus camera at objects nearer or farther away in the scene, the plane of critical focus moves closer to or farther from the camera. As the plane moves, various objects at different distances from the camera come into or go out of critical focus.

Depth of Field

A lens can only bring objects at a single distance from the camera into critically sharp focus. But if you look at photographs, 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 the most sharply focused plane appear acceptably sharp. This area in which everything looks sharp is called depth of field. Objects within the depth of field become less and less sharp the farther they are from the plane of critical focus. Eventually they become so out of focus that they no longer appear sharp at all.

Often it doesn’t matter so much exactly what you are focused on. What does matter is whether or not all of the objects you want to be sharp are within the depth of field so they appear sharp. If you want a large part of the scene to be sharp, you can increase the depth of field. You can decrease it if you want less of the scene sharp. In some scenes, you can significantly increase or decrease the depth of field simply by shifting the point on which you are focused or by changing the aperture setting.

The near and far limits of depth of field are two planes (B and C), parallel to the plane of critical focus (A). Actually, they 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. Notice that in the diagram the depth of field is not evenly divided. At normal shooting distances, about one-third 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 an object, the depth of field becomes more evenly divided.

Plane of Critical FocusThe plane of critical focus in your image will be the area that falls within the focus area in the center of the viewfinder when you press the shutter-release button halfway down.

TipTo control depth of field, switch to aperture preferred mode and select a small aperture for great depth of field or a large aperture for shallow depth of field.

Focus Settings

Most digital cameras have an autofocus system that automatically adjusts the focus to make the subject in the center of the viewfinder appear critically sharp. However, some cameras provide other ways to focus and this is a good thing. Autofocus often has trouble focusing in scenes with little contrast, when the object in the focus point is brighter than the rest of the scene, when the subject is poorly illuminated, when both near and distant objects fall within the focus point, or when the subject is moving quickly. If the camera can’t focus, some cameras beep or blink a lamp. If this happens, use focus lock to focus on a subject at the same distance. Some cameras also let you switch to manual focus.

Using focus lock

To change the position of the plane of critical focus, you can use a procedure called focus lock. Most digital cameras have a two-stage shutter-release button. When you press it down halfway, it sets focus, exposure and white balance. Some cameras beep and illuminate a lamp when these readings are locked in. If you don’t release the shutter-release button you can then point the camera anywhere else in the scene and the settings remain unchanged. This lets you set the focus at any distance from the camera to control both focus and depth of field.

TipTo control depth of field, switch to aperture preferred mode and select a small aperture for great depth of field or a large aperture for shallow depth of field.

How To: Using Focus Lock In record mode, point the camera so the item you want to lock on is in the focus area in the center of the viewfinder. Press the shutter-release button down halfway and hold it there to lock in the focus.  Without releasing the shutter-release button, recompose the scene and press the shutter-release button the rest of the way to take the picture.

A small aperture gave enough depth of field to keep both foreground and background figures sharp.