A variable focal length, or zoom, lens is designed so the focal length can be changed by mechanically moving the elements within the lens. The movement of lens elements, in unison and in precise order, gives a smooth change of image size while maintaining acceptably sharp focus throughout the entire adjustment. The simplified drawing of a zoom lens (fig. 1-38) illustrates how the movement of elements within the lens can

change the focal length while maintaining correct lens-to-film distance.

While only the shortest and longest focal lengths for this particular lens are shown in the drawing, various other focal lengths are possible.

The biggest advantage of a zoom lens is that you have many focal lengths in one single lens. You do not have to change lenses to use a different focal length. Sometimes it is impossible to change your viewpoint to improve a picture. But with a zoom lens you can zoom in and out (change focal length) until you get the exact image you want. One disadvantage is the extra bulk and weight of the zoom lenses. There is also some loss in picture quality when compared to the performance of a fixed focal length lens. There are four basic types of wide to telephoto zoom lenses for 35mm cameras:

Wide-range zoom lenses have focal lengths from about 28mm to 80mm. They often take the place of fixed focal-length lenses of 28mm, 35mm, 50mm, and 80mm.

Mid-range zoom lenses have focal length that do not extend very far on either side of a normal lens focal length. Mid-range zooms for 35mm cameras have a

Figure 1-38. Variable focal-length (zoom) lens.

variable focal length from about 35mm to 70mm. They are usually small and lightweight. They are designed to be used instead of a normal focal-length lens, not to replace a range of focal length lenses.

Long-range zoom lenses start at about 35mm and include the popular 100mm or 105mm focal lengths. They can take the place of four, fixed focal-length lenses: the 35mm, 50mm, 85mm, and 105mm.

Telephoto-range zoom lenses have focal lengths that range from about 50mm to 135mm, 90mm to 23Omm, 75mm to 150mm, and even 135mm to 600mm.

The most popular telephoto zoom is from about 80mm to 200mm.

Now you have some basic knowledge of the way light reacts when striking various objects. You also know how it forms an image through a lens. With practice, you will be able to apply these principles in photographic practices. By using various focal-length lenses and aperture settings, you can control image size and depth of field as well as the apparent perspective of your photographs. In the next chapter, light-sensitive materials are discussed. This will enable you to record permanent images.