Film | Cameras | Lenses | Lighting

Lenses

The lens is the eye of the camera. Its function is to bring light from the subject into focus on the film. A camera can have a single lens or a complex set of lenses. Together with the shutter, the lens controls the amount of light that enters the camera.

A Brief History of Lenses

The modern camera’s predecessor, the camera obscura, consisted of a simple pinhole in the side of a room or box. In the 17th century people discovered they could produce a brighter, sharper image by fitting a camera obscura with a convex (outward-curving) lens. The first such lens came from a pair of eyeglasses. Over the next 300 years, interest in telescopes and microscopes led to the development of better and brighter lenses.

With the invention of photography in the 19th century, the need for camera-specific lenses increased, leading to rapid developments in the field of lens making. These developments took place along two fronts: The first was the invention of new types of glass that refracted light more effectively; and the second was the discovery of ways to combine several pieces of glass, or elements, to control optical distortion.

Quality modern lenses are made of many individual elements of ground and polished glass (6 to 14 elements is common). These elements, each of a different shape and purpose, are cemented into groups; each group is then assembled in what is called a lens barrel. On a manually controlled camera, the lens barrel incorporates an aperture ring and a focusing ring. By turning the aperture ring, the photographer adjusts the opening of the lens diaphragm, which determines how much light reaches the film. The focusing ring is used to focus the image on the film plane by changing the distance between the element groups.

Focal Lengths

Camera lenses are categorized according to their focal lengths and maximum apertures. The longer the focal length, the larger the image inside the camera will be. The greater the size of the aperture, the more light the lens will admit. Focal length is the distance from the optical center of the lens to the image formed inside the camera. Because this distance varies depending on how the camera is focused, focal length ratings are defined by measuring the distance when the focusing ring is set for photographing a distant subject (indicated on the focusing ring with the symbol 8, called infinity). A lens with a short focal length is commonly called a wide-angle lens; with a long focal length, a telephoto lens. Lenses that approximate the angle of view of the human eye are called normal lenses.

Focal length determines the magnification and angle of view of the image. With the camera in a fixed position, objects photographed with a wide-angle lens will seem farther away than with a normal lens; seen through a telephoto lens, the same objects will seem closer (and closer together). The wide-angle can take in a broader angle of view than the eye can see, while the telephoto narrows this view.

The zoom lens offers a range of focal lengths, and is one of the most popular types of lenses today. The user can change the focal length by simply pushing a button or turning a ring on the lens barrel. So-called true zooms maintain focus while changing the focal length; this allows photographers with single-lens-reflex cameras to focus precisely at high magnification before framing the picture at a different focal length. Another type of zoom lens, the varifocal lens, must refocus as the focal length changes—a disadvantage only if the camera does not offer automatic focusing.

Macro Lenses

Some photographic subjects require task-specific optics. The most common specialized task is close-up photography, for subjects ranging from flowers to coins. To cope with these small subjects, macro lenses were developed for single-lens-reflex cameras. Macro lenses for 35-millimeter cameras extend the focusing range to a matter of inches. On their own they can reproduce an object on film at one-half its actual size; with the addition of an extension ring, the camera can picture an object at life size.

Many modern zoom lenses come with a macro setting that allows a limited range of close-up focusing. However, these are no substitute for a true macro lens because, at best, they only reproduce an object at one-fifth its actual size. Extension rings or simple close-up lenses also can attach to a normal lens to allow close-ups. Magnification of a subject to greater than its actual size calls for more specialized equipment, such as a microscope, and is called photomicrography.

Aperture

The lens diaphragm controls the size of the aperture, or lens opening, and thus the amount of light that passes through the lens. It operates in conjunction with the shutter. The aperture size is measured by numerical settings called f-stops. On a traditional, manually controlled camera the f-stops are inscribed on an adjustable ring that fits around the lens. Typical f-stops are f/2, f/2.8, f/4, f/5.6, f/8, f/11, and f/16. The setting f/2 represents a large aperture, f/16 a small aperture. With simple automatic-exposure cameras, a computer sets the aperture size; thus the aperture ring has disappeared from many of today's lenses.

Lenses come with a rating for their maximum aperture, indicating how much light can reach the film when the lens diaphragm is wide open. With single-lens-reflex cameras, the maximum aperture also influences how bright the image appears in the viewfinder. Within lens types, a lens with a large maximum aperture will have a larger diameter and weigh more than a lens with a smaller aperture. A telephoto lens requires a larger lens diameter and greater length to let in the same amount of light as a normal or wide-angle lens. Like telephoto lenses, zoom lenses are also physically large. To reduce their bulkiness and complexity, many manufacturers now design zoom lenses with a variable maximum aperture: The size of the aperture changes as the focal length of the lens goes from wide-angle to telephoto settings.

Focusing

Technically, film captures only one plane of a picture in perfect focus. However, in practice we call a picture “in focus” when it appears reasonably sharp at a given magnification and viewing distance. Until recently photographers had to bring an image into focus manually, by turning a ring or a focusing collar on the camera lens. But most of today's cameras with built-in lenses will adjust the lens automatically, through use of a mechanism connected to an autofocusing sensor. Cameras with interchangeable lenses still have focusing collars to allow for manual adjustment. Most lenses will focus from a few feet in front of the camera to a point in the far distance, called infinity.

Depth of Field

To help determine what will appear in focus in a picture, photographers make use of a concept called depth of field. This term refers to a zone of focus—that is, the area between the closest and farthest objects that will appear sharply focused in the photograph. A picture with a deeper zone of focus might be a landscape in which both the trees in the foreground and the mountains in the background appear in sharp focus. A picture with a shallow depth of field might be a close-up portrait, in which objects in the background are purposely blurred.

The factors that determine depth of field are lens aperture, focusing distance, and focal length. All other factors being equal, depth of field will be greatest when photographing a distant subject, using a short focal length (wide-angle) lens, and a small aperture. Conversely, depth of field will be most shallow when photographing a subject at close range, using a long focal length (telephoto) lens, with a wide aperture.

A photographer using a single-lens-reflex camera or view camera can judge the approximate depth of field by looking directly through the lens with the aperture set to the desired f-stop. In cameras with removable, manually adjusted lenses, a depth-of-field scale shows the approximate sharp-focus zone for the different aperture settings.

Automatic cameras are designed to focus precisely on a single subject at the center of the frame or, in more sophisticated designs, to focus on a band of details across the central picture area. In most cases, the photographer locks in the focus by pressing the shutter button part way. For capturing the image of a moving subject, certain cameras with motor drives will adjust the focus continuously while the photographer tracks the subject.

Focusing precisely on a central subject, however, does not necessarily provide the greatest depth of field. With manual focusing, photographers can obtain the maximum depth of field by turning the focusing collar until the infinity sign aligns with the outside depth-of-field mark for the f/stop they have chosen. A variant of this manual-focusing technique is called zone focusing: The photographer chooses an aperture and a focusing distance that together cover the range of distances at which the subject is likely to appear. Zone focusing is especially useful for candid photography.

Lens Hoods and Coatings

One of the worst enemies of photographers is flare, unwanted light that enters the lens and causes strange reflections and a loss of contrast on the film. Flare is especially obvious when photographing with the sun in front of or just to the side of the lens. To decrease the incidence of flare, photographers can shade the front of the lens with a collar called a lens hood that prevents sunlight from striking the glass surfaces. Hoods for zoom lenses are less effective because they must angle away from the lens enough to accommodate the lens's widest angle of view.

Lens makers also combat the more subtle effects of flare by coating the exterior and interior surfaces of the lens’s glass elements with thin layers of reflection-absorbing material. These coatings enhance the contrast of the film image and account for the characteristic green and purple hues visible when one looks into the front of a modern lens.