The origin of f numbers is simply a calculation whereby the focal length of the lens is divided by the diameter of the aperture.

Note about compact cameras

Digital compacts have an aperture control of some kind, but because compacts are physically smaller in size they tend to have smaller sensors and shorter focal lenses than the typical SLR. Because of this, the calculation is smaller. For example, on most compacts an aperture of f4 will provide similar characteristics as f8 on an SLR.

Maximum and minimum apertures differ depending on lens

Note: The smaller the f number (e.g. f1.4, f2 etc.), the larger is the aperture, thus allowing more light reaching the film to compensate for the duration of the shutter curtain travelling time.

FIG. 1.10 Each consecutive f-stop lets in half as much light as the preceding f-stop.

1/15th sec can sometimes be momentarily confused with 15 seconds. The majority of SLR cameras indicate full seconds with inverted comas after the number, i.e. 4'' indicates 4 seconds.

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