of light reaching the film or sensor, in accordance with the Weber–Fechner law describing a logarithmic response of the human visual system to light. A single stop of exposure is one unit on a base- logarithmic scale. More precisely, the exposure value of a photograph is defined as where is the f-number measuring the aperture of the lens during the exposure, and is the number of seconds of exposure. Binary logarithms (expressed as stops) are also used in densitometry, to express the dynamic range of light-sensitive materials or digital sensors. Calculation Conversion from other bases An easy way to calculate on calculators that do not have a function is to use the natural logarithm () or the common logarithm ( or ) functions, which are found on most scientific calculators. To change the logarithm base from or to one can use the formulae: or approximately Integer rounding The binary logarithm can be made into a function from integers and to integers by rounding it up or down. These two forms of integer binary logarithm are related by this formula: The definition can be extended by defining . Extended in this way, this function is related to the number of leading zeros of the 32-bit unsigned binary representation of , . The integer binary logarithm can be interpreted as the zero-based index of the most significant bit in the input. In this sense it is the complement of the find first set operation, which finds the index of the least significant bit. Many hardware platforms include support for finding the number of leading zeros, or equivalent operations, which can be used to quickly find the binary logarithm. The fls and flsl functions in the Linux kernel and in some versions of the libc software library also compute the binary logarithm (rounded up to an integer, plus one). Iterative approximation For a general positive real number, the binary logarithm may be computed in two parts. First, one computes the integer part, (called the characteristic of the logarithm). This reduces the problem to one where the argument of the logarithm is in a restricted range, the interval , simplifying the second step of computing the fractional part (the mantissa of the logarithm). For any , there exists a unique integer such that , or equivalently . Now the integer part of the logarithm is simply , and the fractional part is . In