What are the lens size, focal length and F-number?

 

Lens size

A lens’ size is related to the diagonal size of the image sensor used. As examples, a 1/2-inch lens is for use with a sensor having an 8.0-mm diagonal, not a 12.7-mm diagonal, and a 1/4-inch lens is for use with a sensor having a 4.0-mm diagonal, not a 6.35-mm diagonal. This table relates the lens sizes to actual sensor diagonals.

 

Focal Length

This parameter indicates how far behind the principal plane of a lens an image will form, as well as the magnification obtained from the lens for a given imaging condition. A lens with a long focal length produces images far behind the lens’ principal plane, as well as high magnifications and narrow fields of view (like a telephoto lens). A lens with a short focal length produces images close behind the lens’ principal plane, as well as low magnifications and wide fields of view (like a wide-angle lens).

F-Number

This is the “focal ratio,” the ratio of the lens’ focal length to its diameter (or aperture). It usually ranges from f/1.4 to f/8. Smaller f-numbers let in more light, while larger f-numbers allow more latitude for focus. The amount of light let in by the lens is inversely proportionate to the square of the f-number. This means that an f/8 lens lets in 16x less light than an f/2 lens. The iris of some lenses provides control over the f-number or effective lens diameter. In low-cost applications, however, the f-number is usually fixed and it is preferable to have an f/1.4 or f/2 lens so that the sensor receives

 

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