April 03, 2008
This is part of an ongoing series of short articles about lenses and how they are used on Digital SLR cameras. The series currently contains the following articles:
What is normal?
A normal lens is one that creates an image that is very close to how our eyes perceive the world. In other words, images made with these lenses “look normal.” There are no distortions introduced, or apparent compression of space that you might get with other lenses.
What is normal for a given camera depends on the size of whatever is doing the imaging. The formal definition is a lens whose focal lenght equals the diagonal of the image size. So a view camera with a 4" × 5" back has a diagonal of 154mm (96 × 120mm), and therefore a normal lens would be a 150mm lens. A large format 8 × 10 camera would have a 300mm normal lens.
In the days when film ruled, a 35mm camera used film that was 36mm × 24mm 1. This image has a diagonal of 43mm. This number was rounded up to 50mm—a compromise between the theoretical value and good lens sharpness. So this explains why a 50mm lens is often referred to as a normal lens.
However, most digital SLRs use sensors that are slightly smaller than the size of 35mm film. These sensors have image diagonals of 27mm. But the lenses they use are still designed to create an image on a piece of 35mm film. What happens is that the image that is created is effectively a crop of what the image would have been if it were a full frame or film camera. The ratio of full frame to the smaller sensors comes out to 1.6, and many times these cameras are referred to as having a 1.6 crop factor.
1So why is it called _35mm_ if neither dimension is actually 35mm? The width of the film itself is 35mm. Each roll has perforations along the top and bottom of the image area (the 24mm dimension) that allow the film to be advanced. The extra width taken up by these perforations, make the total width of the film 35mm.