The specifications for a given lens typically include the maximum and minimum aperture sizes, for example, –. In this case, is the maximum aperture (the widest opening), and is the minimum aperture (the smallest opening). The maximum aperture opening tends to be of most interest and is always included when describing a lens. This value is also known as the lens "speed", as it affects the exposure time. The aperture is proportional to the square root of the light admitted, and thus inversely proportional to the square root of required exposure time, such that an aperture of allows for exposure times one quarter that of .