Special Lenses: Digital Camera Zoom

A zoom is a lens of variable focal length , therefore the photographer has in his hand various types of lenses. Depending on the focal lengths is covers, a zoom can be angular, normal or telephoto, two or three things at a time. The focal length of this type of lens can vary in a continuous way between the two ends of the interval it covers. Used at first in cinema and television, zoom lets us instantly regulate the size of the image without the need of moving the camera or changing the lens. In this way, it allows is to see the change in size when changing the focal length, which causes another form of movement on the screen. On the other hand, in each photograph we can only choose on focal length, which will give the image determined size and perspective characteristics.

Zoom was been progressively imposing in compact cameras, and of course digitals. In reflex cameras the constitute an option that is being used more by customers. It is relatively rare in medium format cameras and inexistent for large format. Almost all the new small format and digital cameras come with a "universal" zoom, with a vision angle that goes from angular to short telephoto. As a lens for a reflex, the zoom covers the functions of many lenses at a time, which lets us avoid changing the lens every time we want to change the focal, and also saves money, space and weight in comparison to the set of lenses it replaces. Its continuous focal allows us a more precise and rapid control of the frame. This is specially useful when making slides, were there are few possibilities to modify the final frame of the final image after shooting. Some manufacturers express the range of a zoom as an increase factor similar to what we have seen in telephoto, although the meaning is not the same. If a zoom for 35 mm format covers focal longitudes of, for

example, 30 to 150 mm, we can see that the manufacturer advertises 5 x, but this factor only expresses the relation between the minimal and maximal focal lengths; nonetheless, the maximum increase factor (150 mm) regarding the normal focal length (50 mm) that we have seen for telephotos is of 3.

The greatest inconvenience of the zoom is it's lower luminance. Since luminance is the relation between focal length and the diameter of the maximum aperture, in a zoom with a large coverage we can find that the luminance is reduced while we pass from the lower end to the higher.

During a long time the most frequent has been to use lenses of fixed focal and zoom has been an exception, above all for great angle lenses and normal lenses. Zoom was limited to the field of telephotos, were it is important to be able to vary the amplification factor. This is still true in medium format cameras. Actually, in small format and in digital photography it is the contrary: zoom is the law and fixed focal lenses are the exception. This can also have an inconvenience: fixed focal lenses force us to look for a frame more consciously, to move until finding the ideal point of view. A change in the focal is a deliberate act because it forces us to dismount the lens we have on the camera and substitute it for another. On the other hand, with a "universal" zoom we run the risk of having the bad custom of making pictures from any point of view and only influencing the size of the image with the focal, forgetting or ignoring perspective issues..