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== Photography == In photography, aperture, exposure, and film speed generally follow powers of 2: The aperture size controls how much light enters the camera. It is measured in [[F-number|f-stop]]s: {{f/|1.4}}, {{f/|2}}, {{f/|2.8}}, {{f/|4}}, etc. Full f-stops are a [[square root of 2]] apart. Camera lens settings are often set to gaps of successive thirds, so each f-stop is a sixth root of 2, rounded to two significant digits: 1.0, 1.1, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.5, 2.8, 3.2, 3.5, 4.0, etc. The spacing is referred to as "one-third of a stop". (Rounding is not exact in the cases of {{f/|1.2}}, {{f/|3.5}}, {{f/|5.6}}, {{f/|22}}, etc.) The [[film speed]] is a measure of the film's sensitivity to light. It is expressed as ISO values such as "ISO 100". An earlier standard, occasionally still in use, uses the term "ASA" rather than "ISO", referring to the (former) American Standards Association. Measured film speeds are rounded to the nearest preferred number from a modified Renard series including 100, 125, 160, 200, 250, 320, 400, 500, 640, 800... This is the same as the R10β² rounded [[Renard series]], except for the use of 6.4 instead of 6.3, and for having more aggressive rounding below ISO 16. Film marketed to amateurs, however, uses a restricted series including only powers of two multiples of ISO 100: 25, 50, 100, 200, 400, 800, 1600 and 3200. Some low-end cameras can only reliably read these values from [[DX encoding|DX encoded]] film cartridges because they lack the extra electrical contacts that would be needed to read the complete series. Some digital cameras extend this binary series to values like 12800, 25600, etc. instead of the modified Renard values 12500, 25000, etc. The [[shutter speed]] controls how long the camera lens is open to receive light. These are expressed as fractions of a second, roughly but not exactly based on powers of 2: 1 second, {{frac|1|2}}, {{frac|1|4}}, {{frac|1|8}}, {{frac|1|15}}, {{frac|1|30}}, {{frac|1|60}}, {{frac|1|125}}, {{frac|1|250}}, {{frac|1|500}}, {{frac|1|1000}} of a second.
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