Focal Length

Vincent Bockaert, 123di.com

The focal length of a lens is defined as the distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the subject (at infinity) is "in focus". The camera lens projects part of the scene onto the film or sensor. The field of view (FOV) is determined by the angle of view from the lens out to the scene and can be measured horizontally or vertically. Larger sensors or films have wider FOVs and can capture more of the scene. The FOV associated with a focal length is usually based on the 35mm film photography, given the popularity of this format over other formats.

In 35mm photography, lenses with a focal length of 50mm are called "normal" because they work without reduction or magnification and create images the way we see the scene with our naked eyes (same picture angle of 46°).

Wide angle lenses (short focal length) capture more because they have a wider picture angle, while tele lenses (long focal length) have a narrower picture angle. Below are some typical focal lengths:

Typical focal lengths and their 35mm format designations
< 20mm
Super Wide Angle
24mm - 35mm
Wide Angle
50mm
Normal Lens
80mm - 300mm
Tele
> 300mm
Super Tele

A change in focal length allows you to come closer to the subject or to move away from it and has therefore an indirect effect on perspective. Some digital cameras suffer from barrel distortion at the wide angle end and from pincushion distortion at the tele end of their zoom ranges.

35mm Equivalent Focal Length

Focal lengths of digital cameras with a sensor smaller than the surface of a 35mm film can be converted to their 35mm equivalent using the focal length multiplier.

Optical Zoom (X times zoom) and Digital Zoom

Optical zoom = maximum focal length / minimum focal length
For instance, the optical zoom of a 28-280mm zoom lens is 280mm/28mm or 10X. This means that the size of a subject projected on the film or sensor surface will be ten times larger at maximum tele (280mm) than at maximum wide angle (28mm). Optical zoom should not be confused with digital zoom.

This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
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