THE PHOTOGRAPHIC EYE

Fast & Slow
Besides being available in different
lengths, lenses also differ in terms of
the apertures they can accommodate.
You might hear a lens referred to as,
for example, a 50mm f/2.8 ("fifty
millimeter, f two eight"). The first
number refers to the focal length of
the lens and the second number refers
to its largest aperture.
As you know, larger apertures let
in more light. If an image is correctly
exposed in 1/60 of a second with an
aperture of f/4, it will take only
1/125 of a second to be correctly
exposed at f/2.8. With a larger
aperture, the film is exposed more
quickly.
So, a lens with a very large maximum
aperture (f/2 or f/1.4, for example)
is called a fast lens. One with
a relatively small maximum aperture
(say, f/4) is called slow. Fast lenses
allow you to shoot at faster speeds or
in lower light than slow lenses do.
Unfortunately, increasing the speed
of a lens also increases its price. This
is particularly true for telephoto
lenses.
The aperture number is calculated
by dividing the focal length of the
lens by the diameter of the lens opening.
The actual lens opening of a
200mm lens would therefore have to
be four times as wide as that of a
50mm lens to achieve the same maximum
aperture:
200mm focal length
4- 71.4mm diameter = f/2.8
50mm focal length
^ 17.8mm diameter = f/2.8
17.8mm x 4.01 = 71.4mm
As you might guess, increasing the
amount of optical-quality glass and
other materials also increases the cost
of producing a lens. However, a telephoto
lens —even a fast one —is rarely
useful in low light. To get the best
results, a telephoto needs a fairly fast
shutter speed. It also needs as wide
a depth of field as possible, since it
is difficult to hold telephotos steady
and focus them accurately. Therefore,
you can get by just fine with a
fairly small maximum aperture (f/4
and f/3.5 are common and affordable).
With normal and wide angle
lenses, large apertures become far
more useful and, fortunately, less expensive.
It's a good idea (though by
no means necessary) to have at least
one short lens with a maximum aperture
of f/2 or f/1.4.
Perspective 139