Electronic Photography

Video starts with a camera, as does all picture taking. In still and motion-picture film photography, there is a mechanical system that controls the amount of light falling on a strip of film. Light is then converted into a pattern of varying chemical densities on the film.
As a physical medium, film can be cut, spliced, edited, and manipulated in other ways as well. In electronic photography, the light from an object goes through a lens, as it does in film photography. On the other side of the video camera lens, however, light is converted to an image by an electronic process as opposed to a mechanical or chemical process. The medium for this conversion has changed over the years. It began with tube cameras and has progressed to completely electronic components. The tube cameras will be discussed first, followed by a discussion of the same process as it occurs in digital cameras.

Tube Cameras
In a video tube camera, the lens focuses the image on the face of a pickup tube inside the camera. The face of the pickup tube is known as the target (Figure 2.1). The target is light-sensitive, like a piece of film. When light shines on the face of the target, it conducts electricity in proportion to the amount of light that is striking its surface. Without light on the face of the target, the target resiststhe flow of electricity.

A stream of electrons, called the beam, comes from the back end of the tube and scans back and forth across the face of the target on the inside of the pickup tube. The electrical current generated is either allowed to pass from the target to the camera output or not, depending on the amount of resistance at the face of the target. The amount of resistance varies depending on how much light is shiningon the target. In every video camera, there are adjustments for the beam intensity and the sensitivity of the face of the target. The target acts as either an insulator, when it’s not exposed to light, or as a conductor when light shines on its face. The electrical signal that flows from the target is, in effect, the electronic recreation of the light coming from the scene at which the camera is aimed.

Scanning the Image
Scanning the image begins with the beam of electrons sweeping back and forth across the inside face of the target. Where the electron beam strikes the face of the target, it illuminates an area the same size as the electron beam. This “dot” of electron illumination is called the aperture (Figure 2.2).


The dot or aperture is the smallest size that an element of picture information can be. The larger the aperture, the less detail in the picture. The smaller the aperture, the more detail in the picture. Dot size, or beam aperture, is comparable to drawing with large, blunt crayons or fine-tipped pens. Crayons can outline shapes or color them in. A fine-tipped pen can add texture and small highlights toa drawing. In a digital video signal, these picture elements are called pixels, short for picture elements (Figure 2.3).
The electron beam must always be kept perpendicular to the face of the target. If it were not perpendicular as it scanned back and forth, then only one line in the center of the television image would be in focus. The lines closest to the top and bottom of the picture would be badly distorted, because at these angles the aperture dot would be shaped like an ellipse.