How lenses work

The lens acts as the eye of your camera, and, like the human eye, some lenses focus better than others. Some are better at seeing distances; others are better at reading fine print. Camera lenses are typically made of many individual polished glass elements. For the camera, the function is the same: to focus light rays onto a light-sensitive surface, or in this case, a digital sensor. The better the lens does the job, the sharper your picture. The simplest type of lens is a single-element convex lens, the same kind used in a magnifying glass. In profile, it is thick in the middle and tapered at the ends, as in 4-1.

A lens takes incoming light and focuses it on a single point of convergence, called the focal point; from there, the light continues on until it strikes the image sensor

The lens is the optical component of the camera, the part that actually absorbs and bends light. At its simplest, a lens is just a curved piece of glass; at its most complex, it involves numerous elements that are measured, ground, and arranged with subatomic precision. In either case the purpose is the same: to gather beams of light reflecting from an object and focus them to form a real image — an image that looks like the scene in front of you.

How a lens works is surprisingly simple. As light travels from one medium to another, it changes speed. Light travels more rapidly through air than glass, so a lens slows the movement of light waves. When light enters a piece of glass at an angle, one part of the wave reaches the glass before another and consequently starts slowing down. As the light wave enters the glass at an angle, it bends, or refracts, in one direction. It bends again when it exits the glass because parts of the light wave enter the air and speed up before other parts of the wave. In a standard or convex lens, one or both sides of the glass curve out. This means rays of light passing through the lens bend toward the center of the lens on entry. The image is turned upside down after passing through the lens; this is accounted for by your camera so that you see a normal image through the viewfinder.

A lens with a rounder, more convex shape (a center that extends out farther) has a more acute bending angle. Basically, curving the lens out increases the distance between different points on the lens. As a result, the light makes a sharper turn which means that wide, normal, and tele-photo lenses bend light differently. The wider the angle, the more the light must bend through the lens to reach the image sensor — and, consequently, why wide-angle photos look so much more distorted.

In 4-2, notice how an ultrawide-angle fisheye lens bends and distorts this image of a pickup truck, while in 4-3, even a normal 50mm lens — which is capable of taking very nondistorted, natural portrait — bends and distorts light when focused extremely close to the subject, although to much less of a degree.

The photo of the pickup truck in 4-2 was taken with an EOS 5D and a Canon 15mm f/2.8 Fisheye lens at f/8. ©Jim White. The photo of the swimmer in 4-3 was taken with an EOS-1Ds and a Canon 50mm lens at f/5.

At a basic level, convex lenses force light to converge at a specific point, which is called the focal plane. From there, the light travels to your image sensor. At that point, the image is cast upside-down, which the camera’s firmware flips right-side-up before displaying on your LCD. Similarly, your camera’s mirror flips the image so that it looks right-side-up when viewing it through the viewfinder.

Lenses are designed so that the distance from the focal plane to the image sensor is exactly correct in order for the image to fit the sensor’s size — which, for most detachable lenses, assumes that it is the same as a 35mm film frame or a full-frame image sensor. Most digital cameras, however, have image sensors smaller than full-frame. As a result, part of the image is cropped out (the crop factor).

The distance from the front of the lens, where light enters it, to the focal plane, where light converges, and then from the focal plane to the image sensor, is much longer on a telephoto lens than on a wide-angle lens. When you adjust the zoom ring on a zoom lens, you change the configuration of the lens elements to alter the focal length to accommodate the distance between lens elements. Your zoom lens contains multiple glass elements (each a lens itself) that work together to magnify and converge light in just the right way so that your image passes through the lens precisely.

The magnification power of a lens is determined by focal length. A longer focal length means greater magnification. Using different lenses allows you to photograph the same scene with different angles of view, such as seen in 4-4 and 4-5. While their focal lengths aren’t all so very much different (one ranges from 16 to 35mm, taken at the 16mm position, and the other is 15mm), the lens design, configuration, and elements are nonetheless significantly different— creating an equally significantly different effect.

This moving carousel in the first image was shot with an EF 16-35mm f/2.8L lens; in the second, an EF 15mm f/2.8 Fisheye lens was used. 4-4 taken at ISO 50, 1/1 second, f/18; 4-5 taken at ISO 100, 1/13 second, f/8. Both images taken with an EOS 5D.

Telephoto, normal, and wide-angle lenses are suited to different situations and subjects. If you’re taking a picture of a mountain range, you may want to use a telephoto lens with an especially long focal length in order to see distant subjects closer up. Telephoto lenses allow you to focus in on specific elements in the distance, so you can create tighter compositions. If you’re taking a close-up portrait, you might use a normal lens. This lens has a shorter focal length and wider angle of view, but won’t let you magnify distant objects.

A standard 50mm camera lens doesn’t significantly magnify or shrink the image, making it ideal for shooting objects that aren’t especially close or far away and providing a natural look and feel. Compare the same image when shot with a normal 50mm lens and a telephoto 200mm lens (see 4-6 and 4-7).

These photos were taken with the same camera (an EOS 5D) and at the same exposure (ISO 320, 1/60 second, f/22), but with different lenses: 4-6 was taken with an EF 50mm f/1.4 prime lens (a normal lens), and 4-7 was taken with an EF 70-200mm f/2.8L zoom lens fully extended to a 200mm focal length.

Category: Science of Lenses

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