What is the focal length of a lens?

The focal length of a lens is one of the factors determining its “power” of magnification. This distance is always expressed in millimeters (mm), except on some very early lenses built in the U.S. or the U.K.

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Here are a few simple examples:

28mm Lens = Wide Angle Lens

50mm Lens = Normal  lens

100mm+ Lens  =  Telephoto Lens

Crop Factor

However, these numbers are true ONLY if the light sensor of our camera is the size of a 35mm film frame, 24 x 36 mm. Here’s an example:

film 35mm

Click to enlarge: 35mm format and coverage offered by different sized sensors (red)

Why? Because there is a relationship between the size of the sensor and the focal length of the objective, ratio that influences on the image magnification. Without going into too much technical detail, let’s just say that the smaller the sensor, the larger the magnification for a given focal length. So what is a wide angle for 35mm size sensor can become a normal focal length for a smaller sensor and even a telephoto lens for a very small sensor. See the red boxes in the image above, examples of the coverage offered by two smaller sensor formats compared to 35mm.

Most modern cameras, except models for advanced amateurs and professionals, use a much smaller sensor than 24×36 mm. The focal lengths of the objectives will therefore also be significantly smaller.

Common sensor sizes:

Please active images: Various sensor sizes. Image © 2015 Charles Martel

Click to Enlarge: Various sensor sizes. Image © 2015 Charles Martel


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