How to Use Zoom Lenses
Zoom lenses are lenses with variable
focal length. A zoom lens is designed in such a way that one or more of
its glass elements can be moved with respect to one another, so that
its focal length can be continuously changed throughout a fairly wide
range. The usual ranges are from somewhat wide angle to somewhat
telephoto, from very short to moderate wide angle, from moderate wide
angle to normal, or from a short to a longer telephoto. This can be a
great convenience when the photographic situation is changing so
rapidly that there is little or no time for changing lenses.
Zoom lenses can be divided for
convenience into four general categories. The most versatile in some
ways are those that combine wide angle, normal, and telephoto focal
lengths, such as a 35-105 mm lens. Next handiest are the short to
longer telephoto zooms, typified by a 70-205 mm range. Less often seen,
but sometimes very useful, are those that feature focal lengths that
range from wide angle to normal, such as 24-50 mm lens. There is also
the macro zoom lens, typically a short to longer telephoto lens with a
close focusing capacity.
Zoom lenses can generally be used much
like other lenses, but they give you the unique capability of
infinitely adjustable image size, within their individual zoom and
focusing range. This is useful in any form of photography, but is
especially so in color slide work, where you cannot adjust the image
size or composition, after the fact, in the darkroom.
Additionally, zoom lenses offer a further
unique capability: you can change the lens focal length, and hence the
image magnification, over the period of a time exposure, thus radially
smearing the image.
Zoom lenses often pose certain characteristic problems. They are
frequently subject to substantial internal reflections that tend to
introduce "flare" (a lowering of image contrast), and if the lens is
pointed toward, or nearly toward, a light source such as the sun, this
will possibly cause single or multiple bright spots or pseudo images on
the film for film photography. Flare is significantly reduced and often
eliminated by closing the lens diaphragm one or more f-stops from
maximum. Actual bright spots from light sources must be watched for any
time you are working into the light. It is sometimes possible to use
them as creative image elements if you see them and integrate them into
your composition. But if you fail to notice them while composing, they
may well spoil your picture.
In times past, no zoom lens could offer
as much image resolution (picture sharpness) as a fixed focal length
lens of comparable focal length. Currently, however, with advances in
lens design and especially the widespread introduction of computer
designing coupled with the use of special rare earth glasses, the
general quality of zoom lenses have greatly improved.
The best new designs are very good
indeed, and they offer a degree of operational flexibility and
versatility unmatched by more conventional equipment.
In compact photography, zoom lenses are
the most commonly seen. Although fixed focal length lenses are still
providing better quality in most cases but it has become limited to