A measure of the ability of a lens to gather light. Also called its “speed”. The ratio of the focal length of the lens to its effective aperture.

The number resulting when the focal length of a lens is divided by the diameter of the aperture. A sequence of f-numbers calibrates the aperture in regular steps (know as stops) between the minimum and maximum openings of the lens. The f-numbers generally follow a standard sequence, in such a way that the interval between one full stop and the next represents halving or doubling in the image brightness. Also called f-stop or relative aperture. The f-number becomes progressively higher as the aperture is reduced to allow in less light; i.e. f/5.6, f/8, f/11, f/16, f/22, f/32... Theoretically, all lenses at the same f-number produce images of equal brightness.

a number that equals the focal length of a lens divided by the diameter of the aperture at a given setting. Theoretically, all lenses at the same f-number produce images of equal brightness. Also called f-stop or relative aperture.

This is used to indicate image brightness as formed by the lens and controlled by the iris. The smaller the f-number, the brighter the image is.

Indicates the brightness of the image formed by the lens, controlled by the iris. The smaller the F-number the brighter the image.

The ratio of the focal length of the lens to its optical diameter. Smaller F-number lenses allow more light through; the phrase â€œstopping down the lensâ€ refers to increasing the F-number, thereby allowing less light in and allowing the thermal imager to image objects at higher temperatures.

Found on an adjustable camera, this is an indication of the size of the lens opening. Common f numbers include f/1.4, f/2, f/2.8, f/5.6, f/8, f/11, f/16 and f/22, noting that the larger the f/number the smaller the lens opening. F numbers work together with shutters speeds to determine exposure settings on the camera.

The focal length of lens divided by its usable diameter. In the case of a laser the usable diameter is the diameter of the laser beam or a smaller aperture which restricts a laser beam.

Expression denoting the ratio of the equivalent focal length of a lens to the diameter of its entrance pupil (smaller F/# provides larger aperture of the lens, transmitting greater amount of light).

A number indicating the brightness of the image and controlled by the iris. The smaller the f-number, the brighter the image.

A scale used to express the relative area of the aperture of a lens, simply the result of dividing the focal length of a lens by the effective aperture of the lens opening (the apparent size of the diaphragm seen from the front of the lens). The f-number increases by the multiple of the square root of 2, or 1.4142, from 1.0, 1.4, 2.8, 4, 5.6, 8, 11 and so on, allowing each to pass half the light of the aperture below and twice the light of the aperture above in the scale.

The f-number indicates the brightness of the image formed by the lens and is controlled by the iris. A smaller f-number indicates a brighter image.

In lenses with adjustable irises, the maximum iris opening is expressed as a ratio (focal length of the lens)/(maximum diameter of aperture). This maximum iris will be engraved on the front ring of the lens.

a measurement of the lens aperture; the smaller the number the bigger the aperture.

The ratio of focal length to aperture for a lens assembly.

The numbers on the lens aperture ring and the camera's LCD (where applies) that indicate the relative size of the lens aperture opening. The f-number series is a geometric progression based on changes in the size of the lens aperture, as it is opened and closed. As the scale rises. each number is multiplied by a factor of 1.4. The standard numbers for Calibration are 1.0,1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, etc., and each change results in a doubling or halving of the amount of light transmitted by the lens to the film plane. Basically, calculated from the focal length of the lens divided by the diameter of the bundle of light rays entering the lens and passing through the aperture in the iris diaphragm.

Another label for focal ratio.

The f-number or focal ratio of an lens is the diameter of the lens opening in terms of the effective focal length of the lens. Common f-numbers are f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, and f22. The greater the f-number, the smaller the aperture and the less light will pass the shutter.

A symbol that expresses the relative aperture of a lens.

A number that indicates the size of the lens opening on an adjustable camera. The common f-numbers are f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, and f/22. The larger the f-number, the smaller the lens opening. In this series, f/1.4 is the largest lens opening and f/22 is the smallest. Also called f-stops, they work in conjunction with shutter speeds to indicate exposure settings.

Optical or lens speed. Smaller f-number means faster lens.

A set of numbers used to express the aperture of a lens that represents its light transmission. It is calculated by dividing the focal length of the lens by the diameter of the lens diaphragm opening. Also known as a "stop" of "F stop."

The number associated with the f-stop. large number is a tiny opening small number is a large opening

In optics, the f-number (sometimes called focal ratio, f-ratio, or relative apertureSmith, Warren Modern Lens Design 2005 McGraw-Hill) of an optical system expresses the diameter of the entrance pupil in terms of the effective focal length of the lens. It is the quantitative measure of lens speed, an important concept in photography.