What is a "normal" lens in photography?

When a photographer talks about a "normal" lens, he or she is basically referring to a lens that generates a photograph that looks as close to what the human eye naturally sees. That's in contrast to a wide-angle or telephoto lens, that have shorter or longer focal lengths respectively.

However, a normal lens renders only perspective the same way the human eye sees, but not field of view. This seems to confuse some people because the field of view of a human is almost 180º, which in photography will require a really wide-angle lens. However, a lens like this will significantly change perspective, making background objects appear much farther away than they really are. So, in photography a normal lens is one that generates a normal perspective but has no relation to field of view.

How to calculate a "normal" lens?

In traditional 35mm film cameras, a normal lens is usually the 50mm. However, a 50mm is a wide-angle lens in large format cameras. For example, for a camera that uses 8x10 sheet film, the normal lens is 300mm! It's important to remember that the size of the recording surface (be it film or a digital sensor) has a direct impact on what a normal lens is. So how do we know for our camera?

A normal lens is one in which the focal length is equal to the diagonal size of the recording surface.

As an example, 35mm film and a full frame digital sensor are 36mm x 24mm, making the diagonal 43mm. Since there are no 43mm lenses, the closest one is considered the normal. Hence, the 50mm.

Most digital SLR's use a smaller sensor called crop sensor. Nikon calls it a DX and its size is 23.7mm x 15.8mm. Using this one, we can see that the normal lens is 28mm. This means that a 50mm will be slightly telephoto on a crop sensor.

If we do the same with Point & Shoot cameras, which have an even smaller sensor, we can easily get the relevant normal lens. A camera like the G10, with a 1/1.7" sensor, will require a 9.5mm focal length to give us a natural perspective.

Because 35mm has been the most widely used film size for years, most photographers are used to quoting focal lengths in 35mm terms. That's why focal lengths are often referred to as 35mm equivalent. So, in a point & shoot camera with a 1/1.7" sensor, 9.5mm will be the 35mm equivalent of a 50mm lens.