Understanding Focal Length
Every lens has a focal length. You are probably familiar with the terms
Because many people are familiar with 35 mm, digital cameras state their focal lengths in 35 mm equivalent terms. On a 35 mm camera, a lens with a focal length of less than 35 mm is known as a short, or wide-angle, lens. One with over 65 mm is considered a long, or telephoto, lens. And lenses with focal lengths between 35 mm and 65 mm are considered normal. The 50 mm lens is the most common lens.
Focal length is a primary tool in the photographer's tool box. A wide-angle lens, for example, can produce very distorted images when objects are close to the lens. This can be a great artistic effect or a terrible mistake depending on your intentions. Telephoto lenses magnify backgrounds and bring them up to appear closer behind a subject.
Magnification goes hand in hand with angle of view. A short lens, with its wide angle of view, requires all the objects in a scene to be reduced in size in order to fit into the image sensor. It has the effect of pushing the subject away from you. Conversely, the long lens, with its corresponding narrow angle of view, will have the effect of pulling objects in a scene close to you, causing them to appear larger.
Today most cameras come with a zoom lens, which is a variable focal length lens. Inexpensive ones may have a 3″ zoom, which means that the longest focal length is three times the shortest. More-expensive cameras may have a 10″ or more zoom range.
While choosing a digital camera, the first question to ask yourself is how you plan to use your camera. If you want to photograph landscapes, buildings, and interiors, you will want wide-angle capability. If you are interested in shooting portraits or nature scenes, telephoto capability should work. A middle ground would be a normal lens setting between 35 mm and 65 mm (35 mm equivalent).
Working distance is the distance between the photographer and the subject. A telephoto lens, for example, allows more working distance and therefore is better for portraiture. A subject will feel more comfortable when the camera is not up close. A wide-angle lens, on the other hand, allows very little working distance when taking a picture up close.
This is a wide-angle shot of a person standing on a dead-end road.
This shot is taken with a normal lens from the same position as the previous wide-angle shot.
This telephoto shot is taken from the same place as the previous wide-angle and normal shots.
To take a sharp picture, use the right shutter speed for the particular focal length. For example, because a telephoto lens is magnifying an image, it is also magnifying any camera shake. A good rule of thumb is that the shutter speed should be about the same as (or higher than) the focal length in a 35 mm equivalent. For example, a 1/30 of a second shutter speed is good for a 35 mm lens, 1/60 of a second for a normal or 50 mm lens, 1/125 of a second for a zoom of about 100 mm, and 1/250 second for a zoom of 200 mm.
To control shutter speed, change the automatic exposure setting to shutter priority, which will allow you to pick the shutter speed. To control aperture or depth of field (described later in this chapter), change your exposure setting to aperture priority, which will allow you to pick the f-stop.
