Learning about Lenses
A lens in its simplest form is a pretty basic thing: a piece of transparent material, usually glass, with two polished surfaces that modify rays of light. Before the advent of SLR photography, most cameras had built-in lenses that captured images similar to what people saw when they looked through the viewfinder. Such things as making far-away images seem closer or larger wasn't possible with these lenses.
The ability to make images larger and smaller and to have greater control over such aspects as depth of field, angle of view, and perspective is what made SLR cameras popular when they were first introduced, and what continue to be key factors in why more photographers use them than any other camera. The manor house and grounds in the color insert were shot with a standard lens.
Today, there are four basic types of lenses for SLR cameras:
Normal or standard: Objects are similar in size to what your eyes can see (approximately 50mm for 35mm SLR cameras).
Telephoto: Makes faraway objects appear closer.
Wide-angle: Covers a wider angle of view than is possible with a normal or telephoto lens.
Zoom: Combines the features of the first three types in one lens. Some zoom lenses operate strictly in the telephoto area. For example, a common zoom lens size is 100mm–300mm, which even at 100mm is going to enlarge what you see.
What distinguishes each type of lens is a factor called focal length. Expressed in millimeters — such as 50mm — focal length describes the distance from the optical center of the lens to the film. It also determines the scale of the images in a picture. In general, lenses with short focal lengths make images look smaller; as focal lengths get longer, image sizes get bigger. If you were to stand in the same place and take a picture using lenses with different focal lengths, you would get the widest angle of view and the smallest images with the shortest focal length lens. As you went up in focal length, images in the scene would look bigger, but you would see less of the scene itself as the angle of view narrowed.
FIGURE 9-1 In this photo of a cruise ship in Newport's harbor taken with a wide-angle lens, much of the surrounding area is included in the shot as well.
FIGURE 9-2 This photo was taken with a telephoto lens from the same spot as the previous one. Note how much larger the image of the cruise ship is and how much more it dominates the scene.
You can usually check the focal length of a lens by looking at the front lens ring, although some lenses have their length engraved elsewhere on the lens barrel. If the focal length of a lens is about the same as the diagonal of a frame of the film the camera uses, it's called a normal lens. If it is longer, it's a telephoto; shorter, and it's a wide angle. The diagonal of a 24mm × 36mm image (the size of the image on 35mm film) is 43mm, so a lens with a focal length of 43mm or thereabouts is considered a normal lens for the 35mm format. If you're using a camera with larger or smaller film, the focal lengths used to classify its lenses will be different.
Focal length also determines how much of a specific scene or area the lens can see. This is called field of view or angle of view, and it is measured and expressed in degrees. Lenses with short focal lengths have wide angles of view and cover more of a given scene. As focal lengths get longer, angles of view get narrower and you can see less from the outer edges of the scene.
Focal length and field of view are two of the most important factors to consider when choosing which camera lens to use. Together, they determine how much of a particular view you'll be able to capture and what the size of the images will be.
Always use a lens that will let you fill as much of the frame as you can with your intended subject. The pros call this “cropping in the camera.”Doing so will deliver much better images than those that are enlarged and then cropped in the regular or digital darkroom.
