19.1 Simple Camera

The simplest camera is a pin-hole camera, just a box with a pin-hole in one end of it to allow some light to come in as sketched in Figure 19.1. At the end opposite this pin-hole we can place a piece of film to record the image or a ground glass to view the image from behind the camera. (You can make such a pin-hole camera yourself; a piece of waxed paper makes a very adequate substitute for a ground glass).

Figure 19.1 A simple pin-hole camera allows in only one ray of the multitude that leave each point on the object.

Look at the rays of light in the sketch of the pin-hole camera in Figure 19.1. From each point on the object there are rays of light leaving in every possible direction. A few of the rays of light coming from the top of a tree are shown in the sketch. Only one of those rays passes through the pin-hole and arrives at the back of the camera. That is true for every point on the object. From each point on the object only one ray of light will pass through the pin-hole. Each of these rays that pass through the pin-hole will have a different direction, as illustrated in Figure 19.2 for rays coming from several different points on the tree. That means each of these rays strikes the back of the camera at an appropriate place to produce a clear, sharp image.

Figure 19.2 A sharp image is produced in a pin-hole camera since only a single ray from each object point arrives at the corresponding image point.

An interesting feature of a pin-hole camera is that it produces a sharp image regardless of the object distance as sketched in Figure 19.3. There, a sharp image is produced from a far distant tree and a nearby face. This is because only a single ray (or a very tiny bundle of rays) from each point on the object passes through the pin-hole and continues on to a unique point on the image.

Figure 19.3 In a pin-hole camera, all objects are focused clearly, regardless of their distance from the camera.

With such a tiny opening or aperture to admit light in a pin-hole camera, the image may be dim. Using a larger aperture (perhaps a "nail-hole" instead of a pin-hole) will make the image brighter. But, with more rays of light from each point on the object, the image becomes blurred. As indicated in Figure 19.4, light from each individual point on the object now creates a blurred circle on the image and these fuzzy dots overlap. The image is now brighter-but blurred.

Figure 19.4 A larger aperture lets in more light-but the image becomes blurred.

A lens in the opening can be used to focus the image as in Figure 19.5. Now the image will be sharp only for a particular object distance do. Usually the focal length of the lens f is fixed. To focus on nearby or far away objects requires that the image distance di be changed; when you focus a camera, you are adjusting this distance between the lens and the film.

Figure 19.5 A lens will allow more light to enter the camera for a brighter image and still keep the image focused (for a particular object distance).

As the aperture of a lens is made smaller, or "stopped down", the camera behaves more like a pin-hole camera and focus becomes less critical. The range of object distances for which the image is reasonably clear is called the "depth of field". Stopping down a lens to a smaller apeture increases its depth of field. This can be seen from Figure 19.6 where the lens is focused to provide a sharp, well-focused image of a distant object on the film. A more nearby object (shown inverted only to keep the two distinct) will have its image somewhere beyond the plane of the film. Each point on the object will have a "circle of confusion" on the film. For a wide aperature the film will record a rather fuzzy, blurred image, but as the aperature is made smaller the "circles of confusion" will be smaller and the film will record a sharper image. This can be used for creative effects by causing something in the foreground or background to blur while the main subject is sharply focused. This can be seen in the photos of Figure 19.7. It must be considered when changing lens setting and shutter speed.

Figure 19.6 "Depth of field" changes with the aperture. A smaller lens aperture provides a large depth of field while a larger lens aperture provides a very small depth of field and requires more careful focusing. The nearby object is shown below the axis merely to keep the two ray diagrams distinct.

Figure 19.7 "Depth of field" refers to the range of object distances that produce a clear image. A smaller lens aperture provides a large depth of field while a larger lens opening provides a very small depth of field and requires more careful focusing.

A typical modern photographic lens is illustrated in Figure 19.8. Such lenses are far more complex than the simple, single-element lenses we discussed in the previous chapter but the general behavior and purpose remain the same.