Camera Focus and Depth of Field

The focusing mechanism (1.5.1) lets us regulate the distance of the lens to the film, to be able to have focused images on different planes of the scene. Every focus distance implies utmost sharpness in one plane; images in other planes will become more blurred the farther away from the selected distance they are. In the strictest terms, there is only on valid position on the focus ring for each distance. In theory, with a tape measure and a focus distance scale on the lens, it's possible to focus with exact precision without visual verification.

Nevertheless, it's not easy to be calculating distances, nor is a precise, well numbered scale always available. Focusing usually takes place without the distance scale but with a more simple method (but subject to errors). Nowadays, cameras have an automatic focusing system ( autofocus , 7.1.3.1), or a viewfinder and/or a telemeter (distance measurer).

On cameras with a viewfinder or a LCD screen , in case of digital cameras, focus is achieved by looking for sharpness in the image. Once we find the object we want to appear in focus, the focus ring is turned until the maximum sharpness is reached in that part of the image.

As additional help, the viewfinder on many reflex cameras comes with a split image telemeter in the middle, were the image will appear broken when there is no focus. Finding sharpness in a viewfinder can be particularly hard in low light conditions. The split image telemeter is suitable for these situations because the eye can spot a slight discontinuity than a slight lack of focus.

In a camera with an optical viewfinder , focus is reached using the superposition of an image double of lower intensity in the middle of the screen with the image we wee through it (double image telemeter). When focus is correct, the double image disappears.