Which f number

They will not always be equivalent because two lenses will have a different amount of light loss as the light travels through the lens. Light loss in a lens is dependent upon the number of elements and various lens coatings in each lens. At infinity focus the detector will be one focal length away from the rear principal plane of the lens. For a given aperture diameter d and focal length f we can calculate the optics F-number:. Every increment in the F-number smaller aperture reduces incoming light by a factor of 2.

This is defined as:. Aperture Aperture controls the brightness of the image that passes through the lens and falls on the image sensor. High f-number increased depth of field To view this content, JavaScript must be enabled, and you need the latest version of the Adobe Flash Player. Depth of field: long Click image to enlarge. Depth of field: short Click image to enlarge. One can define the working f-number based on that for given imaging conditions; it is correspondingly larger than the f-number.

Normally, the f-number of a photographic objective can be changed in certain steps, with typical values like 2, 2. Some objectives offer only relatively large f-number values, because image aberrations could not be properly compensated for lower values. Unfortunately, that limits their light gathering power, which for distant objects is determined by the f-number.

Particularly for close objects, the light gathering power can be reduced substantially. That aspect is relevant for macro photography, where exposure times have to be increased accordingly.

For photography of small objects over short distances with a substantial magnification macro photography , the image brightness is substantially lower than one might expect from the f-number.

It depends on the working f-number , which is larger see above. For astronomical telescopes , the same kind of numbers is usually called the focal ratio or f-ratio.

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