FOCAL LENGTHS
One of the most obvious characteristics of a lens is its focal length or in the case of a zoom lens, its range of focal lengths. This is because a lenses focal length determines its angle of view or how large an area of a scene you can capture. Along focal length (telephoto) has a narrow angle of view and lets you capture smaller subjects farther away such a bird on a tree limb. A short focal length (wide-angle) has a wide angle-of-view and lets you capture a wide area such as a room interior or landscape.

Focal lengths by themselves don't tell you much. What photographers care about is how much of a scene they can capture. The problem is that at any given focal length, this depends on the size of the image sensor. Since different cameras have differently sized image sensors camera companies have a difficult time describing their lenses. They know their lens is 7.1mm but most photographers have no idea what that focal length means on that particular camera. All they know is that on a 35mm camera that would be a fisheye lens. To make focal length designations more meaningful, digital camera lenses are compared to 35mm camera lenses with which many photographers are already familiar. Experienced photographers know that a 35mm lens on a 35mm camera is wide-angle and a 80mm lens is telephoto.

For a digital camera with a built-in lens, you are given the actual focal length of the lens and then an equivalent focal length on a 35mm camera that would capture the same area in a scene. For example, a camera may list its lens as 7.5mm (equivalent to 50mm on 35mm camera). Zoom lenses will give you a range of focal lengths. For example a lens may be listed as 7.1mm- 23mm (35-105mm equivalent in 35mm photography).

Cameras with interchangeable lenses are often adapted from 35mm film cameras and use the same lenses as the film versions. These lenses project an image circle onto the image sensor that is designed to cover a frame of 35mm film. When used on a digital camera, the angle of view captured in the image depends on the size of the camera's image sensor. If the digital version of the camera uses an image sensor that's the same size as a frame of 35mm film, called a full-framesensor, then a lenses angle of view is the same as it is on a film version of the camera. If the image sensor is smaller than a frame of film, as many are, it will capture a smaller area, effectively increasing the lenses focal length. Effective focal lengths can increase by a factor of 1.5 x or so compared to the indicated focal length of the lens. As a result, a lens that is 100mm on a film camera will be l50mm on the digital version. This multiple works across the entire family of lenses that work with the camera, making wide-angle lens less so on a digital camera, and making telephoto lenses more so.

Digital cameras with interchangeable lenses designed from the ground up for digital photography are just appearing. The Four Thirds (4/3) system introduced by Olympus and Kodak establishes a open standard so lenses can be swapped between any 4/3 camera regardless of who makes them.

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