The focal length is the distance between the lens and the focal plane - that is, the surface of the film - when the lens is focused on infinity. At that distance light rays reflected off a subject reach the focal plane in parallel lines. The focal length determines the angle of view seen through the lens as well as the size of the image formed on the film.

A normal lens has about a 47 degree angle of view. This view is similar to the way our eyes would see without looking through the camera.

This is the same scene viewed through a short, or wide angle, lens; a 28mm. Although I have not changed my position, the scene appears to be much further away. Also, the angle of view has increased to about 73 degrees, so we see more of the background.

Now the same scene through a longer than normal lens; a 135mm. The scene appears much closer because the lens forms a larger image on the focal plane. The angle of view is much narrower; only about 23 degrees.

What about different cameras? Is a 55mm lens the "normal" lens for a medium format camera or a 4x5 view camera? The answer is no because a lens is considered normal if the focal length is about the same as the diagonal measurement of the film.

Thus, a 43mm lens is normal for a 35mm camera. However, most 35mm SLR cameras are fitted with a 55mm lens as the standard or normal lens. Actually, lenses ranging from 40mm to 55mm should be considered normal. A camera that uses a 2 1/4 x 2 1/4 format is usually fitted with an 80mm lens as its normal one. And a 4x5 view camera takes a normal lens in the vicinity of 150mm. Because a lens is considered normal does not necessarily mean that it should become your most frequent choice. A photographer needs to experiment with various lenses in different photographic situations in order to learn the advantages and disadvantages of each.

Short lenses, like 28mm, will give us a greater angle of view than a normal lens, and it will give a greater depth of field; that is, a deeper area of acceptable focus from foreground to background.

Although I focused on the model in the foreground, the model behind him is also in focus, as is the distant background. The extreme depth of field enhances the three dimensional feeling of the picture. But this apparent depth can sometimes present problems in composition.

In this picture the lens has placed a very distracting background in focus. This characteristic of wide angle lenses brings up an important point in regard to camera vision. Photographers must learn to see what's going on in the background elements. They must learn to neatly compress the chaotic, three-dimensional world onto the two-dimensional viewfinder. One simple technique that can be used to soften the effect of distracting backgrounds is to use the aperture as a controller of depth of field.

This picture was shot at f16, which maximizes the range of focus for this lens.

But if we use the largest aperture, f2.8, we will greatly reduce the depth of field, throw the background out of focus, and make the foreground much easier to read. Another characteristic of short lenses that you should be aware of is distortion, both real and apparent.

Real distortion is caused by the thick pieces of glass used in wide angle lenses and is evident at the edges of the picture. Straight lines will appear curved near the outside of the frame. This effect becomes more pronounced on even shorter lenses. So if you use a wide angle lens for architecture, for example, you should avoid placing verticals near the edges. The apparent distortion that frequently occurs with short lenses is one of perspective. A short lens is capable of focusing very close to the subject, which can cause distortion of the perspective.

If you switch to a longer lens, like 105mm, 135mm or 200mm, you'll have to move further away from the subject to be able to focus the image. Because of their narrow angles of view, long lenses can be used to draw attention to the important details of your scene. They also seem to compress space.

Long lenses are a good choice for a casual portrait situation, like we have here, since the camera is placed at a comfortable distance from the subject. Disadvantages of long lenses include their considerable bulk and weight. So if you hand-hold the camera and use a slow shutter speed, you may see evidence of camera movement. You'll probably need to shoot at 1/250th second or faster to avoid blurring your pictures. Better yet, you might want to use a tripod.

If you're not interested in collecting several different short and long lenses, you have the option of using a zoom lens which incorporates a variety of focal lengths, from short to long, in one lens. The main advantage of the zoom lens is that, without changing your position, you can alter the framing of your subject. The disadvantages of the zoom lens include its greater size and weight, and that image quality may not be as high as with individual lenses, because a zoom requires several more lens elements. These additional elements can soften the overall image quality.

As we have seen, there are many photographic design choices associated with lenses. What angle of view do you want? How much depth of field do you need? How do you handle distortion? Or compression of space? You simply must experiment with several lenses, learn their idiosyncrasies, and you will soon appreciate the range of options they provide.