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[1] [2] [3] The f-number is also known as the focal ratio, f-ratio, or f-stop, and it is key in determining the depth of field, diffraction, and exposure of a photograph. [4] The f-number is dimensionless and is usually expressed using a lower-case hooked f with the format f / N , where N is the f-number.
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.
A scene will be properly illuminated for every combination of f‑stop and distance shown on this flash device's exposure calculation dial, each of which equals the same guide number. The guide number here (full power setting, ISO 100, and normal-angle coverage) is 37 for calculations made in meters (yellow arrow) and 120 for feet (orange).
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 ...
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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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