When learning photography with a single lens reflex (SLR) camera, one of the first things a photographer notices is the amount of information on the lens. Most lenses contain several pieces of information in numeric form. These pieces of information are critical to selecting a lens, as they determine how the finished image will look.
F-Stop
Also called the aperture, the f-stop settings will be the number closest to the camera body on the lens. These settings determine how wide the lens aperture opens when the shutter release is pressed, allowing more or less light to fall on the sensor. The "speed" of the lens is determined based on how wide the aperture can open--a faster lens will allow the aperture to open wider than a slower one. Smaller numbers mean a wider aperture and more light but will allow a shallower depth of field, or area that will be in focus. Larger numbers allow a smaller aperture and less light but have a much broader depth of field.
Focal Length
The focal length is the distance between the lens and the image media, such as film or a digital image sensor. It is written in millimeters on the camera lens and may contain a ratio. The distance in millimeters determines the perspective of the image. Lenses with a shorter focal length are called wide-angle lenses, while lenses with a long focal length are called telephoto lenses. Wide-angle lenses allow more of the scene to be a part of the image, where telephoto lenses allow the photographer to focus some distance away.
Zoom Lenses
Zoom lenses are lenses that allow more than one focal length. This ability lets the photographer change how close or far the subject is. Zoom lenses will often have the focal length written as a ratio; for example, a zoom lens that can move from a focal length of 400 mm to a focal length of 100 mm will have the ratio 4:1 written on it. A zoom lens can be either a wide-angle or a telephoto lens.
Prime Lens
The "opposite" of a zoom lens is a prime lens. Prime lenses have a fixed focal length, and so the photographer must physically move in order to compose the image. These lenses will have markings similar to a zoom lens, giving a focal length in millimeters as well as f-stop settings and optionally a distance scale. Many photographers prefer prime lenses over zoom lenses, as the quality of the resulting image is affected by the lens.
Distance Scale
The distance scale gives measurements of distance in feet and meters. When turned, the ring will align with markings on the lens to denote the minimum and maximum depth of field for that particular setting. This measurement is becoming less common, as it is unnecessary on many auto focus cameras and lenses, which focus for the photographer.
Tags: focal length, zoom lens, depth field, allow more, focal length, focal length called, f-stop settings