The Focal Plane

Another important aspect of imaging is to understand how the image is prepared for exposure onto a piece of film or a digital CCD chip. This is typically done through the utilization of a glass lens system. A simple illustration of such a system can be seen below in figure one.

Figure One (Typical glass lens system with the representation of a focal point)

(image courtesy of

From figure one it is apparent that the incoming light rays that hit the lens are bent (refracted) inside the lens and that the rays are all focused at one single point. This point marks the posistion of the focal plane. Essentially, if the lens were pointed at an object, then the object would appear on a flat plane made up of an infinite number of focal points within that plane (i.e. a dicernable image). These numerous focal points come from the fact that the object is not a pinpoint, but rather spread over a region of the lens' field of view. This can be seen more clearly in figure two below.

Figure Two (Lens with entire focal plane being representend)

(image courtesy of

Furthermore, another more familiar example of this can be seen by clicking here.

From figure two and the satellite dish example, it can be seen that the object infront of the lens is displayed with great sharpness and clarity upon the focal plane. From this, it should become apparent that all that is needed to obtain an image is to place the photosensitive material (film or CCD) at the posistion of this focal plane, and the image would then successfully be recorded.

Another important aspect of this focal plane, as mentioned previously, is that it is formed from an infinite number of focal points. This brings up a very important goal in imaging technology; to make a type of sensor with an infinite number of "light wave recording" zones. Of course, this infinite ideal is impossible in the real world, however, a very large finite number of light recording zones can be created on a given surface area. The number of these recording zones on a particular piece of film or CCD is typically referred to as its resolution. The higher the number of recording zones, the higher the resolution and the higher quality the recorded image will be. In the case of CCD's, these recording regions are called pixels and in the case of film, they are typically referred to as its crystal density (however, a crystal is directly analogues to a pixel).

Each one of these recording zones "saves" a portion of the type of light that hits it. To see how this information is saved for both film or CCD's, check out the next two sections about CCD's and Film.