Focal Length in Lenses
by Jim Doty, Jr.
The focal length of a lens determines its angle of view. Short focal length lenses have a wide angle of view and are called "wide angle" lenses. Moderate focal length lenses have a
"normal" angle of view. Long focal length lenses have a narrow angle of view and are often called "telephoto lenses".
The focal length of a lens is usually stated in millimeters (abbreviated "mm"). The
higher the number, the longer the focal length and the narrower the angle of view. A 150mm lens is longer in focal length and has a narrower angle of view than a 75mm lens no matter what format of camera it is on.
The angle of view also changes with the format of the camera. A 50mm lens is a wide angle lens on a medium format camera, it is a normal focal length lens on a 35mm camera, and it is a moderate telephoto
lens on a digital camera with a 1.6x field of view crop. No
matter the format, the longer the focal length, the narrower the angle of view.
Prime lenses are single focal length lenses. 20 or more years ago, a well equipped 35mm photographer would have 2 or 3 wide
angle lenses, a normal lens, and 2 or 3 telephoto lenses. A typical collection might have included these focal lengths: 20, 28, 50, 85, 200, and 300mm.
Zoom lenses have increased in popularity over the last 20 years. Each zoom lens covers a range of focal lengths. My favorite, moderate weight, three lens set from my 35mm days
included 20-35mm, 28-135mm, and 100-300mm lenses. This gave me every focal length from 20mm to 300mm in a simple package.
The popularity of 35mm
films cameras over the last several decades means many photographers think of focal lengths and angles of view in 35mm terms. 40-60mm lenses have a "normal" field of view, 28-35mm lenses are moderate wide angle lenses,
20-24mm lenses are wide angle, and 15-17mm lenses are extreme wide angle. 70 to 100mm lenses are short telephoto lenses, 128-200 mm lenses are medium telephotos, 300-400mm lenses are long telephotos, and 500-1200mm
lenses are extreme telephotos. (These are my definitions of terms. Someone else's may vary.)
The advent of digital SLRs is changing all of this so you have
to divide the typical 35mm film focal length by the field of view crop of the D-SLR to get the equivalent focal lengths.
The group of photos below were all taken from the same spot with a digital SLR with a
1.6x field of view crop. They illustrate the relationship between focal length and angle of view. Each photo is labeled with the actual focal length used ("Digital") and the equivalent 35mm lens that would give
the same angle of view on a 35mm film camera ("35 Film").
FOCAL LENGTH ILLUSTRATIONS