Understanding Lenses

Many digital cameras come with zoom lenses so you can zoom in or out to meet different photographic opportunities. Zoom in on a subject and you can capture distant action at sporting events or in the field. Zoom out and you can capture a wide-angle view of a large group, a roomy interior, or of an expansive landscape. The ability to change your angle of view as you frame your image is one of your most powerful creative controls.

Modern camera lenses are designed on computers, ground to critical tolerances, coated with chemicals to improve light transmission, and then mounted in precision barrels and mounts. These lenses have excellent speed and sharpness, much more so than lenses of just a few years ago. The primary function of a lens is to gather light reflecting from a scene and focus that light as sharply as possible onto the image sensor in the camera. A high-quality lens does this very well, but to get the most out of what it has to offer you should know a few of its characteristics and how they affect your images.

Although your camera is equipped only with a zoom lens, in this chapter we look at the effects it has when used as a normal, wide-angle, and telephoto lens. This approach gives you the background you need to use the lens effectively and creatively.

How a Lens Works

Surprisingly, lenses are not actually needed to take a picture. You can make a camera out of a shoebox with a small hole in one end. Known as a pinhole camera, this primitive device can actually focus an image and record it on film. To make a photograph, the box is loaded in the dark with a light-sensitive film or paper and the pinhole is covered with opaque tape. Peeling the tape back (much like a shutter) to uncover the pinhole (much like a lens aperture) begins the exposure, recovering the pinhole ends it. The exposed film or paper can be removed in a darkroom and the image developed.

Light is bent when it passes between substances having different densities. You can see this if you look at an object that is both in and out of water; for example a spoon in a glass half full of water looks bent at the point where it enters the water. Obviously, the spoon isn’t bent; the light reflecting from the spoon is, as it passes from the dense water to the less dense air. The same effect occurs when light passes from the air through a piece of glass. If the glass is curved correctly, as it is in a camera lens, it can bend the light in such a way that an image of the scene in front of the lens is focused behind it.

The focal length of any lens is the distance between the optical centre of the lens and the point at which it focuses an image. When you use a magnifying glass to focus the light from the sun onto a piece of paper, the area illuminated by the beam will become larger or smaller as you change the distance between the magnifying glass and paper. At the point where the bright circle of light is smallest (and where it might set the paper on fire), the simple lens that constitutes the magnifying glass is in focus. The distance between the magnifying glass and the paper is the lens’ focal length.

Lens focal lengths are based on the physical characteristics of the lens so they are absolute values. However, a given focal length lens may be a wide angle lens on one camera and a telephoto lens on another. This is because descriptions such as "wide-angle" or "normal" depend on the size of the film or image sensor being used. As these get smaller, a given focal length lens magnifies more. There are currently a number of differently sized image sensors used in digital cameras. For that reason, different focal lengths are needed to give the same image coverage on different cameras. Because of the confusion this causes, most digital camera companies give the actual focal length of their lenses and then an equivalent focal length were the lenses to be adapted to a 35mm camera. For example, a camera may list its lens as 7.5mm (equivalent to 50mm on 35mm camera). Because digital equivalents vary widely, we often use the more familiar 35mm focal lengths in this book.

Zoom Lenses

A zoom lens lets you choose any focal length within the range the lens is designed for. When you change focal lengths by zooming the lens, two important effects are immediately obvious in the lens’ angle of view and its magnifying power.

Angle of view refers to how much of a scene the lens covers. Zoomed out, you have a wide-angle of view that captures a wide expanse of a scene. As you zoom in, the field of view narrows and you can isolate small portions of the scene without moving closer to the subject.

Magnification is related to the lens’ angle of view. Since zooming out includes a wide sweep of the scene, all of the objects in the scene are reduced to fit into the image. Zooming in gives a much narrower angle of view, so objects in a scene appear larger.

Zoom lenses on digital cameras work much like those on camcorders. There are two buttons or a movable lever. Pressing one zooms in, increasing the focal length and narrowing the angle of view. Pressing the other zooms out, decreasing the focal length and widening the angle of view.

Normal Zoom

A "normal lens" for a 35mm camera usually refers to a lens with a 50mm focal length. On a digital camera, an equivalent lens will have a much smaller focal length because image sensors are much smaller than 35 mm film. When you zoom your lens and look at the image on the LCD monitor, the scene looks about the same as it does to the unaided eye. Looking at the LCD monitor with the lens zoomed all the way out makes everything appear closer than it actually is. With it zoomed out to a wide-angle, everything looks farther away.

A normal-focal-length (50mm) lens isn’t necessarily the one photographer's normally to use. Many photographers prefer the wider angle of view and greater depth of field provided by a slightly shorter focal length.

Wide-Angle Zoom

Zooming out gives you a wide-angle of view that lets you capture a wide expanse of a scene. This wide angle of view is ideal for use in tight spaces, such as when photographing landscapes and in small rooms where you can’t position the camera a great distance from the subject.

A lens zoomed to a wide-angle also has great depth of field. This great depth of field makes short lenses good for street or action photographs. When out to capture quickly unfolding scenes, keep the lens zoomed out to a wide angle so you’ll have maximum depth of field when you respond quickly to a photo opportunity.

Short lenses also let you focus very close to your subject, and the effect this can have on the perspective in your images can be dramatic. Objects very close to the camera loom much larger than those farther in the background. This distortion in the apparent size of objects can deliberately give emphasis and when carried to an extreme, give an unrealistic appearance to a scene.

In addition to zooming your lens all of the way out for wide-angle coverage, some cameras have wide-angle lens adapters that widen it even more.

Telephoto Zoom

A lens zoomed in on a subject acts somewhat like a telescope: It magnifies the image of your subject. This is especially useful when you can’t get close to your subject—or don’t want to. Zooming in like this is ideal for wildlife, portrait, and candid photography, whenever getting close to a subject might disturb it.

When you zoom in on a subject, depth of field gets shallower so you must focus carefully. Also, zooming in visually compresses space, making objects in the scene appear closer together than they actually are.

The primary drawback of zooming in is that it gives you a smaller maximum aperture. This smaller maximum aperture may require a longer shutter speed and since a long lens magnifies movement, just as it magnifies the subject, you may have to use a tripod instead of hand-holding the camera.

For a telephoto view, you can zoom the lens all the way in. For even more magnification, some cameras have optional lens converters that give you even longer focal lengths.

Optical and Digital Zoom Lenses

Zoom lenses come in two varieties; optical and digital zooms. An optical zoom lens actually changes the amount of the scene falling on the image sensor. Every pixel in the image contains unique data so the final photo is sharp and clear. A digital zoom lens uses sleight of hand by taking a part of the normal image falling on the sensor and then enlarging it to fill the sensor. It does this by adding new pixels to the image using interpolation. The interpolated image doesn’t have as many unique pixels as one taken with an optical zoom so is inferior. In fact, you don’t even need this zoom feature because you can get exactly the same effect just by cropping a normal image in a photo-editing program and then enlarging it.

Portraits with a Zoom Lens

A zoom lens is an excellent portrait lens, especially for head-and-shoulders portraits. When zoomed in you can keep your distance and still fill the viewfinder frame with the subject. Keeping at a distance eliminates the exaggerated perspective caused by working very close to a subject with a shorter focal length lens. It also helps relax your subjects if they get uneasy, as many people do, when a camera comes close.

Perspective: How a Photograph Shows Depth

A photograph can appear to compress space so that objects appear closer together than you expect. Another photograph of the same scene can seem to expand space so that objects appear farther apart than normal. These apparent distortions in perspective—the appearance of depth in a photograph—are often attributed to the focal length of the lens being used but are actually caused by the distance of the lens from the subject.