Focal Length
MMs
Lenses have focal length measured in millimeters (mm).
A 500mm lens has only one focal length.
It has a fixed focal length of 500mm.
A zoom lens, such as a 18mm to 200mm, has all of the focal lengths between those two focal lengths.
Wide-angle
Focal Lengths
A focal length less than 75mm with most digital SLR cameras is wide angle.
The lens can "see" more of a scene from left to right.
Telephoto
Focal Lengths
If a focal length is greater than 75mm, the lens is telephoto.
The lens is like a telescope.
Examples
In the photographs below, the camera was not moved.
Note how the yellow bush and orange flower are in the center of each photograph.
18mm (28mm)*
50mm (75mm)
70mm (105mm)
450mm (300mm)
* The second figure is for film cameras.