The Important of Lenses

Good photographers know their cameras and how they work. This
chapter explains the science behind the most important part of that
equipment: lenses. The more you know how to work with lenses, the
better a photographer you will be. Fortunately, even beginners have
been exposed to what this chapter covers:
■ Focal length. You probably have heard the numbers surrounding
lenses, such as a 28mm lens and a 400mm lens. You might
have heard the terms wide-angle and telephoto used instead of
these numbers.
■ Aperture and depth of field. Open the lens to f2.8 and see
depth of field disappear.
■ Focus. Focus by hand or use autofocus.
■ Special lenses. Use special lenses for close-ups, panoramas, and other special
images.
Carpenters must know their tools to stay in business. The same can be said for professional
photographers. Besides your creativity, nothing influences the quality of
your imagery more than the lens. Studying the characteristics of lenses will make you a better photographer.

Lens Focal Length

The most significant difference among lenses is their focal length. A lens is often
described in terms of its focal length (such as a 50mm lens) or its physical length
(short, medium, long). Technically, focal length is the distance between the lens’s rear
nodal point and the focal plane when the lens is focused on infinity (a far distance
from which light reaches the lens in more or less parallel rays).
The focal length controls magnification, or the size of the image formed by the lens.
The longer the lens, you will find the size of objects in the image greater. A lens of
longer focal length bends light rays less than a short lens does (see Figure 4.1). The
longer the focal length, the less the rays are bent, the farther behind the lens the
image is focused, and the more the image is magnified (see Figure 4.2). The size of
the image increases in proportion to the focal length. If the subject remains at the
same distance from the lens, the image formed by a 50mm lens will be twice as big
as that from a 25mm lens.
Focal length also controls the angle of view, the amount of the scene shown on a given
size of film. A long-focal-length lens forms a larger image of an object than a short
lens. As a result, the long lens must include on a given size of film less of the scene in
which the object appears. If you make a circle with your thumb and forefinger and
hold it close to your eye, you will see most of the scene in front of you—the equivalent
of a short lens. If you move your hand farther from your eye, the circle will be
filled by a smaller part of the scene—the equivalent of a longer lens. You will have
decreased the angle of view seen through your fingers. In the same way, the longer
the focal length, the smaller the angle of view seen by the lens.
The effect of increasing focal length while keeping the same lens-to-subject distance
is an increase in magnification and a decrease in angle of view. Because the photographer
did not change position, the size of the objects within the scene remained the
same in relation to each other. Less is visible in the frame as lenses increase in focal
length.

Normal Focal-Length Lenses

A normal focal-length lens, also called a standard focal-length lens, approximates
what the human eye sees. One of the greatest modern photographers, Henri Cartier-
Bresson, who described the camera as “an extension of my eye,” almost always used
a normal lens. The angle of view of most of his images is about the same as what
the eye can see clearly from one position, and the relative size of near and far
objects seems normal.
A lens that is a normal focal length for one camera can be a long focal length for
another camera (see Figure 4.3). Film size determines what will be a “normal” focal
length. The larger the size of the film format, the longer the focal length of a normal
lens for that format. If the focal length of a lens is about the same as the diagonal
measurement of the film (broken line), the lens is considered “normal.” It collects
light rays from an angle of view of about 50°, the same as the human eye.
Usage varies somewhat. For example, lenses from about 40mm to 58mm are also
referred to as normal focal lengths for a 35mm camera.
A lens of normal focal length has certain advantages over lenses of longer or shorter
focal length. Normal focal length lenses have the following characteristics:
■ Faster. They open to a wider maximum aperture, so they can be used with
faster shutter speeds or in dimmer light than lenses that do not open as wide.
■ Less expensive. The lenses are the least expensive of all lenses because they
are manufactured in such large quantities.
■ More compact. Normal focal length lenses are often the smallest lenses
available. If you’re traveling with a 35mm camera, a 50mm is a must-have.
■ Lighter. These lenses often weigh less than even wide-angle lenses!
What you choose for your lens suite is a matter of personal preference, however.
Some photographers habitually use a shorter focal length because they want a wide
angle of view most of the time; others prefer a longer focal length that narrows the
angle of view to the central objects in a scene. If you have a 35mm camera, a 50mm
lens is a good focal length to start.