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Fast & Slow

Besides being available in different

lengths, lenses also dif fe r 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 maxi-

mum aperture (f/2 or f/1.4, for ex-

ample) is called a fast lens. One wi th

a relatively small ma xim um 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 open-

ing. 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 max-

imum aperture:

200mm focal length

4- 71.4mm diameter = f/2.8

50mm focal length

^ 17.8mm diameter = f/2.817.8mm 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 tele-

photo lens eve n 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. There-

fore, you can get by just fine with a

fairly small maximum aperture (f/4

and f/3.5 are common and afford-

able). With normal and wide angle

lenses, large apertures become far

more useful and, fo rtunately , less ex-

pensive. It's a good idea (though by

no means necessary) to have at least

one short lens with a maximum aper-

ture of f/2 or f/1.4.

Perspective 139

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feet, making distant objects appear

larger and, therefore, closer. They

range from 55mm to 500mm and

beyond. Moderate telephotos (rang-

ing from about 70mm to 150mm) are

often referred to as "portrait" lenses,

since they are most flattering for

faces. The most common portrait

lens length is probably 135mm. Other

popular telephotos include 150mm,

175mm and 200mm. Longer tele-

photos are generally used only for

such specialized work as sports

photography.

(Note: Many photographers use a"doubler" o r , more proper ly, an

"extender" to increase the focal

length of a telephoto lens when nec-

essary. A doubler is a short tube or

lens which, as its name suggests,

doubles the focal length of any lens.

A 150mm lens with a doubler, for ex-

ample, can produce the same results

as a 300mm lens, at far less expense.)

True telephotos (anything over

150mm) present some chal lenges thatyou should know about at this point.

We discussed one of these challenges

earlier: decreased depth of field. If

you like the effect it produces, some

reduction in depth of field can be

perfectly all right. You must focus

more carefully with a long lens than

with a short one, however. If you

don't, you may not have anything in

focus at all.

Another consequence of a longlens that we've already discussed is

reduced angle of view. Because a long

lens has a narrow angle of view, any

motion will be more conspicuous

with a long lens than with a short

one. Once again this can be good or

bad, depending upon wha t you want.

If you want some blurred motion, a

telephoto lens can achieve it.

One of the great drawbacks of long

lenses is that they magnify camera

A wide-angle lens has a wide field of view, producing an apparent

distortion of objects close to it. This can be very effective, if that's the

effect you want. Used to photograph distant objects, however, most wide-angle lenses produce an entirely "normal-looking" image. (Student

photograph by Daniel Watson.)

140 The Photographic Eye