camera: Focusing and Composing the Scene

Focusing and Composing the Scene

Except for pinhole cameras, which focus the image on film or viewing surface through a tiny hole, all other cameras use a lens for focusing. The focal length of a lens, i.e., the distance between the rear of the lens (when focused on infinity) and the film or imaging sensor, determines the angle of view and the size of objects as they appear on the imaging surface. The image is focused on that surface (the focal plane) by adjusting the distance between the lens and the surface. In most 35-mm cameras (among the most widely used of modern optical, or film, cameras) and digital cameras this is done by rotating the lens to move it closer to or farther from the focal plane. With twin-lens reflex and larger view cameras, the whole lens and the panel to which it is attached are moved toward or away from the focal plane.

To view the subject for composing (and, usually, to help bring it into focus) nearly every camera has some kind of viewfinder. One of the simplest types, employed in most view cameras, is a screen that is placed on the back of the camera and replaced by the film in making the exposure. This time-consuming procedure is avoided in the modern single-lens (and other) reflex cameras by placing the screen in a special housing on top of the camera. Inside the camera, in front of the film or imaging sensor, there is a movable mirror that bounces the image from the lens to the screen for viewing and focusing, and then flips out of the way when the shutter is tripped, so that the image hits the film or imaging sensor instead of the mirror. The mirror returns automatically to place after the exposure has been made. In rangefinder cameras the subject is generally viewed by means of two separate windows, one of which views the scene directly and the other of which contains an adjustable optical mirror device. When this device is adjusted by rotating the lens, the image entering through the lens can be brought into register, at the eyepiece, with the image from the direct view, thereby focusing the subject on the film or imaging device. Digital cameras typically have a liquid crystal display (LCD) screen and may have an electronic or optical viewfinder as well; some digital cameras have a viewfinder only. The LCD screen allows the photographer to see the image's content before the picture is taken and after, facilitating the deletion of unwanted pictures. An electronic viewfinder effectively reproduces in miniature the image displayed on the LCD screen; the image can be augmented by the camera software in various ways to aid the photographer.

Sections in this article:

Introduction
Development of the Camera
Motion Picture Cameras
Features of Cameras
Controlling the Light Entering the Camera
Focusing and Composing the Scene
Bibliography

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