The Plane of Critical Focus

Saturday, June 13, 2009

One of the most important things to look for in a camera is how well it focuses. This is important because a lens can only bring one part of the scene into the sharpest possible focus. This part of the scene falls on what is called the plane of critical focus. Subjects falling on this plane will be the sharpest part of the picture. You move this plane toward and way from the camera as you change the focal distance.

There are three ways cameras focus: fixed focus, autofocus, and manual focus.

Fixed focus is found on the least expensive cameras, almost all camera phones, and one-time-use cameras.

Manual focus, found on SLRs and some expensive fixed lens cameras, lets you focus by turning a ring on the lens-in many situations this is the best way by far. On point and shoot cameras you often have to use buttons or dials to manually focus.

Autofocus is available on most modern cameras, and on many low-end cameras it's the only kind of focus. When you press the shutter button halfway down, the camera automatically focuses on the center of the scene or some other designated focus area. It's important that the camera do this quickly and accurately.

Autofocus often has trouble focusing on off-center subjects or on scenes with little contrast, when the object in the focus zone is brighter than the rest of the scene, when the subject is poorly illuminated, when both near and distant objects fall within the focus zone, or when the subject is moving quickly. If the camera can't focus, some cameras beep or blink a lamp. If this happens, it's best if your camera lets you use focus lock to focus on a subject at the same distance or switch to manual focus.

The image is only critically in focus within a plane. The formula that describes the relationship between plane of critical focus, lens and film is:

1 / I + 1 / O = 1 / F

...where I is the film to lens distance, O is the distance from the lens to the plane of critical focus, and F is the focal length of the lens.

In a photograph, the area of critical focus is the portion of the picture that is optically in focus. This does not relate to depth of field, which describes acceptable sharpness.

Depth of field extends away from the plane of critical focus. Reducing the size of the aperture will increase the depth of field, but the plane of critical focus will not change.

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