Photo How To

Choosing and Using Lenses

Mike Stensvold  |  Jun 1, 2002

A lens focuses the light rays that pass through it by refracting (bending) them so that they converge at a common point.

 

The focal length of a lens is the distance behind the lens at which the light rays converge when the lens is focused at infinity; it's the distance from the optical center of the lens to the film plane.

 

When a lens is focused on a subject at a certain distance (A), images of more-distant subjects (B) will be focused in front of the film plane (B') and images of nearer subjects (C) will be focused behind the film plane (C').

 

Ideally, a properly focused lens will reproduce a point on the subject as a point on the film. Unfortunately, that is not the case in practice; a point on the subject will be reproduced as a tiny circle (called a circle of confusion) rather than as a true point (A'). As the point of focus moves farther from the film plane, the circle of confusion grows larger. As long as the circles of confusion remain below a certain size, the image will appear sharp to our eyes.

 

A single-element lens cannot focus all colors of light at a single plane, because shorter (blue) wavelengths travel more slowly through the lens than the longer (red) ones do, and so are bent more. This effect is called longitudinal chromatic aberration (top). Lateral chromatic aberration (bottom) is a lateral displacement of oblique light rays by wavelength (color), which means when light rays strike the lens element at an angle, the short ones are focused more toward the edge of the image, while the long ones are focused toward the center.

 

A single-element lens doesn't focus light rays coming through its edges at the same plane as light rays coming through its center, an effect known as spherical aberration. Most of today's multi-element photographic lenses are well corrected for this; stopping the lens down will also reduce it.

 

Astigmatism is the inability of a lens to focus horizontal and vertical lines at the same plane. Multiple-element designs can correct this problem, and stopping the lens down also helps.

 

Light rays passing through a single-element lens at an angle are focused nearer to the lens than light rays going straight through the center. The result is called curvature of field: the image is focused on a curved plane. This presents a problem, because the film is a flat plane. Either the center of the image appears sharp and the edges are soft, or the edges appear sharp and the center is soft. The lens-maker's solution: multi-element lenses. Stopping the lens down also helps.

 

When light encounters an obstacle (such as the lens diaphragm) in its travels, it bends around the obstacle. This is called diffraction, and is why stopping the lens down doesn't necessarily result in sharper images. The smaller the lens aperture, the greater the diffraction.

 

Pincushion distortion reproduces a square as a pincushion shape, with its sides bowing inward. Barrel distortion reproduces a square as a barrel shape, with its sides bowing outward.
A great tool for creative photographers

You can't beat the 35mm SLR for its combination of features, price and performance. And one its best features is its ability to accept a wide range of interchangeable lenses. From superwide fisheye to supertelephoto, macro close-up to soft-focus, fixed-focal-length or zoom, the perfect lens to suit any shooting situation is available for most 35mm SLR camera models. Canon offers lenses from 14mm superwide-angle to 1200mm supertelephoto for its EOS AF SLRs, Minolta offers focal lengths from 16mm full-frame fisheye and 20mm superwide-angle to 600mm supertelephoto for its Maxxums, Nikon offers autofocus lenses from 14mm superwide to 600mm for its AF SLRs (which can also accept a wide range of manual-focus Nikkor lenses), Pentax provides AF lenses from 17-28mm fisheye zoom to 600mm supertele for its AF SLRs (which can also accept manual-focus Pentax lenses from 15mm superwide to 1200mm supertele, and most can accept the 2000mm mirror), Sigma offers focal lengths from 8mm circular fisheye to 800mm supertele for its AF SLRs (and other brands of cameras), and so on.

Why so many lens choices? Because there are lots of shooting situations. While many not-so-serious shooters can get by with a point-and-shoot camera and its built-in 35-80mm (or thereabouts) zoom, serious shooters use a wide range of lenses to handle a wide range of subjects. Action and wildlife shooters like long lenses that fill the frame with their subjects. Close-up buffs use macro lenses that let them move very close to their subjects. Landscape photographers use wide-angle lenses to capture grand scenic vistas (and other focal lengths for different effects). Portrait photographers prefer short telephotos, for pleasing head shots.

This special section of the magazine will introduce you to the different types of lenses available, and show you how they can help in your photography. But first, here's a little general information on lenses.

What Is a Photographic Lens?
The photographic lens is the photographer's window on the world. Physically, it's just a collection of glass or plastic elements held precisely in position in a light-tight tube, with a camera mount on one end and some means of focusing. Creatively, it's probably the most versatile tool available to the photographer. The main function of a photographic lens is to gather light rays from a scene and focus them sharply on the film. But lenses do a lot more. The lens aperture allows you to control the amount of light that reaches the film, and the depth of field. The focal length determines the magnification of the image and the angle of view. And some lenses produce special effects: life-size close-ups, soft-focus, and shifting to correct converging vertical lines in the photo. And did we mention perspective? Well, no—because the lens doesn't actually control perspective, the camera-to-subject distance does, as you'll see later in this section.

The following pages will teach you some things you should know about lenses and their use. And this knowledge will not only help you make better photographs, but it will help you select the lenses that are best suited for the type(s) of photography you do.

How a Lens Works
How does a lens focus the light rays that pass through it? By refraction, which is a fancy word for "bending." When a light ray passes from one medium (air) to another of different density (a lens element), it changes direction, i.e., it "bends." Just how much it changes direction depends on the composition and degree of curvature of the lens, the angle at which the light ray hits the lens and the wavelength of the ray of light.

The focal length of a lens is the distance behind the lens at which images of distant subjects are focused: the distance between the optical center of the lens and the film when the lens is focused at infinity. Focal length is important because it determines the magnification of the image and the angle of view—how much of the scene will appear in the photograph.

When a lens is focused on distant subjects, closer subjects will not be sharply focused. In order to focus on closer subjects, the lens-to-film distance must be extended. You do this by rotating the lens' focusing ring (or, with autofocus cameras, by partially depressing the shutter button to activate the AF system).

When a lens is focused at a certain distance, theoretically a point at that distance will be reproduced as a point on the film. In real life, though, the subject point will appear as a small circle on the film. Points nearer or farther from the lens than the focused distance will appear as larger circles. These circles are called "circles of confusion." As long as they remain under a certain size, these circles appear to our eyes as points. That's what accounts for depth of field (more on this later).

Bending of light rays (refraction) occurs because their velocity changes when they pass from one medium to another. Because short (blue) rays travel most slowly through glass, short rays are bent the most when they pass from air through a glass element. Long (red) rays travel most rapidly through glass and are therefore bent the least. Since light rays of different colors are refracted to different degrees, a single lens element can't focus all colors of light at the same plane. If it focuses green wavelengths at one plane, it will focus longer red ones slightly behind them and shorter blue ones slightly in front of them. The fancy term for this is chromatic aberration. By combining elements of different composition (high-dispersion and low-dispersion glass or other material), lens makers can reduce the effects of chromatic aberration and produce lenses that bring two of the three primary colors into focus at the same plane (achromatic lenses), or all three (apochromatic lenses).

By the way, chromatic aberration affects the sharpness of black-and-white photos as well as color photos, because our subjects are in color even if our film isn't.

Another thing that a single curved lens element cannot do is focus parallel rays passing through its center and its edges at the same plane. Rays passing through the edges are bent more and thus focused slightly in front of rays passing through the center. This is called spherical aberration, and lens designers can correct it by combining multiple lens elements. You can reduce it by stopping the lens down—spherical aberration is most evident at wide apertures.

Another single-element lens problem is coma, which causes the small circles of confusion to appear elongated rather than round at the edges of the image. Still another single-element lens problem is astigmatism, the inability of the lens to focus horizontal and vertical lines at the same plane. Again, multiple elements help reduce the problem, as does stopping the lens down.

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