What about digital cameras?
The CCD arrays on digital cameras are typically much smaller than the imaging area of 35mm film. This means that the focal length that produces a "normal" field of view is much smaller than 50mm.
Unlike 35mm film which is the same regardless of what camera you use, the size of the CCD array varies from camera to camera. This poses a problem. You cannot know whether a focal length is "normal", wide angle or telephoto without knowing the exact size of the CCD array. Unfortunately, even knowing the exact size of the CCD array may not help because sometimes the imaging area doesn't even cover the entire array!
Because many people are familiar with focal lengths of lenses for 35mm cameras, the digital camera manufacturers choose to describe the focal length of their cameras by reference to the focal length that would produce a similar field of view on a 35mm camera.
Consider, for example, a digital camera with a CCD array measuring 8.10mm by 6.08mm. The diagonal measurement of this CCD array would be 10.13mm. A lens with a focal length of 11.7mm would produce a diagonal field of view of 47°, the same as a 50mm lens on a 35mm camera. So for this camera, an 11.7mm lens would be described as having a 35mm equivalent focal length of 50mm.
By describing the lenses this way, the digital camera companies appeal to users' familiarity with 35mm camera equipment.
Are all digital cameras the same?
Unfortunately the digital camera companies do not all use the same method for determining 35mm equivalency. There are at least three methods used by camera manufacturers.
Some camera companies use the diagonal field of view exactly as described in this article. That is, if they describe a camera as having a particular 35mm equivalent focal length, it means the camera produces the same field of view along the diagonal as a 35mm camera with a lens having the stated focal length.
Other companies use the horizontal field of view rather than the diagonal field of view. This produces a slightly different result from using the diagonal because the aspect ratio of the digital image (usually 4:3) is different from the aspect ratio of 35mm film images (exactly 3:2). When a digitial camera produces the same horizontal field of view as a 35mm camera with a lens having a particular focal length it actually has a larger diagonal field of view.
Finally, some companies describe the camera as having a 35mm equivalent focal length of 50mm if the focal length is exactly equal to the diagonal measurement of the CCD array and scale all other focal lengths accordingly. In this way they appeal directly to the idea that 50mm is the "normal" focal length. These cameras actually have a larger field of view in all directions when compared to a 35mm camera with a lens having the stated focal length.
Unfortunately, this means that one camera maker's 50mm equivalent may or may not have the same field of view as another. And you can't compare actual focal lengths without knowing the size of the image on the CCD array.