the camera decide the shutter speed and sometimes also ISO sensitivity for the correct exposure. This is also referred to as Aperture Priority Auto Exposure, A mode, AV mode (aperture-value mode), or semi-auto mode. Typical ranges of apertures used in photography are about – or –, covering six stops, which may be divided into wide, middle, and narrow of two stops each, roughly (using round numbers) –, –, and – or (for a slower lens) –, –, and –. These are not sharp divisions, and ranges for specific lenses vary. Maximum and minimum apertures The specifications for a given lens typically include the maximum and minimum aperture sizes, for example, –. In this case, is currently the maximum aperture (the widest opening on a full-frame format for practical use), 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 . Lenses with apertures opening or wider are referred to as "fast" lenses, although the specific point has changed over time (for example, in the early 20th century aperture openings wider than were considered fast. The fastest lenses for the common 35 mm film format in general production have apertures of or , with more at and , and many at or slower; is unusual, though sees some use. When comparing "fast" lenses, the image format used must be considered. Lenses designed for a small format such as half frame or APS-C need to project a much smaller image circle than a lens used for large format photography. Thus the optical elements built into the lens can be far smaller and cheaper. In exceptional circumstances lenses can have even wider apertures with f-numbers smaller than 1.0; see lens speed: fast lenses for a detailed list. For instance, both the current Leica Noctilux-M 50mm ASPH and a 1960s-era Canon 50mm rangefinder lens have a maximum aperture of . Cheaper alternatives have appeared in recent years, such as the Cosina Voigtländer 17.5mm , 25mm and 42.5mm manual focus lenses for the Micro Four-Thirds System. For