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CHAPTER
NIGHT AND LOW-LIGHT PHOTOGRAPHY / What Gear Is Important

has some descriptive numbers engraved or printed
on the lens barrel. These should include the focal
length and the maximum aperture, as shown in
Figure 3-8.

aperture. Lenses that have a variable maximum

aperture are those where the widest aperture

available changes, depending on the focal length.

When the lens is at the shortest focal length, it

has the widest aperture; as the focal length gets

longer, the aperture gets smaller. These lenses are

very popular because they are much cheaper,

smaller, and lighter than equivalent lenses that

have a constant aperture. The downside to these

lenses is that as you zoom out, you have to change

the shutter speed or ISO settings because the

aperture setting is changing automatically.

Consider a 17-55mm f/2.8 lens. It has a range of

focal lengths from 17mm to 55mm and a maxi-

mum aperture of f/2.8 at all of these focal lengths.

So no matter what focal length you are using,

the maximum aperture available is f/2.8. An

18-55mm f/3.5-5.6 lens, on the other hand, might

be much less expensive than its f/2.8 counterpart,

but there is a really big difference in the size of

the maximum aperture. The 18-55mm lens has

a range of maximum apertures that goes from

f/3.5 to f/5.6 depending on the focal length. At

the shortest focal length, 18mm, the maximum

aperture is f/3.5, and as you change the focal

length, the maximum aperture changes. By the

time the focal length is at 55mm, the maximum

aperture is f/5.6. This change is more than a full

stop of light, so at the longest focal length, there

is less than half the amount of light reaching the

sensor as there is at the shortest focal length.

So when you use one of these lenses you need to

adjust the shutter speed and or the aperture when

using the longest focal length compared to the

shortest focal length.

One lens I use for all my outdoor shooting is the

80-400mm f/4.5-5.6 lens, shown in Figure 3-9.

This lens is great for long exposures, and the wide

Constant- and variable-aperture lenses

There are two different types of zoom lenses:
those that have a constant maximum aperture
and those that have a variable maximum

ABOUT THIS PHOTO The information plate on the 300mm f/4.0
lens showing the focal length and the maximum aperture.

3-8