A wide-angle lens is one that has an angle of view significantly wider than a normal human perspective.
Humans actually have a horizontal field of view of about 180°, but can only see clearly in the central field, which corresponds to about 40°.
To complicate matters, lens manufacturers like to quote the angle of view corresponding to the diagonal of the film format (because this gives a larger number), and the effective angle changes when a supplementary lens is added.
As a rough guide, a `normal perspective' or `standard' lens for use in air has a focal length approximately equal to the width of the film format multiplied (i.e. W x 1.414).
- For the 35mm format (36 x 24mm), the standard lens is 36 x 1.4 = 50mm.
- For square medium format (57 x 57mm), the standard lens is 57 x 1.4 = 80mm.
- For a video camera, a typical format might be 6.4 x 4.8mm, giving a normal view when the zoom lens is set to about 9mm.