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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 ...
Many digital cameras have a display setting and possibly a physical dial whereby the photographer can set the camera to either over or under expose the subject by up to three f-stops in 1/3 stop intervals. Each number on the scale (1,2,3) represents one f-stop, decreasing the exposure by one f-stop will halve the amount of light reaching the ...
The Waterhouse stop or Waterhouse diaphragm is an interchangeable diaphragm with an aperture (hole) for controlling the entry of light into a camera. A thin piece of metal (the diaphragm) is drilled with a hole (the aperture); a set of these with varying hole sizes makes up a set of Waterhouse stops, corresponding to what today we call f-stops ...
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
Lens speed is the maximum aperture diameter, or minimum f-number, of a photographic lens. A lens with a larger than average maximum aperture (that is, a smaller minimum f-number) is called a "fast lens" because it can achieve the same exposure as an average lens with a faster shutter speed. Conversely, a smaller maximum aperture (larger minimum ...