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The f-number N is given by: = where f is the focal length, and D is the diameter of the entrance pupil (effective aperture).It is customary to write f-numbers preceded by "f /", which forms a mathematical expression of the entrance pupil's diameter in terms of f and N. [1]
An example: A deep red filter with an f-stop change of 3 stops = A change of 3 f-stops is equivalent to a filter factor of 8. As a consequence of this relationship, filter factors should be multiplied together when such filters are stacked, as opposed to stop adjustments, which should be added together.
N is the f-number; and; t is the exposure time ("shutter speed") in seconds [2] The second line is just applying the quotient identity of logarithms to the first line. EV 0 corresponds to an exposure time of 1 s and an aperture of f /1.0. If the EV is known, it can be used to select combinations of exposure time and f-number, as shown in Table 1.
Guide numbers may be expressed in either of these units of measure: f‑number⋅meters or f‑number⋅feet. Throughout most of the world where the metric system ( SI ) is observed, guide numbers are expressed as a unitless numeric value like 34 , even though they are technically a composite unit of measure that is a two- factor product : f ...
Increasing f-stop decreases the aperture of a lens. In photography, stopping down refers to increasing the numerical f-stop number (for example, going from f / 2 to f / 4), which decreases the size (diameter) of the aperture of a lens, resulting in reducing the amount of light entering the iris of a lens. [1] [2]: 112
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