ASA number
Quick Summary
In photography, "speed" refers to standards that indicate the size of the lens opening (aperture), the duration of exposure (shutter speed), and the film's sensitivity to light. The aperture, measured in f-stops, controls how much light enters the camera. Shutter speed determines how long the shutter stays open, affecting how motion is captured. Film speed, now universally known as ISO, was formerly referred to as ASA (American Standards Association) or DIN. A higher ISO number signifies greater sensitivity to light, allowing for photography in dimmer conditions or with faster shutter speeds. Sensitometry is the science of measuring this film sensitivity by plotting the film's response to graduated exposures.12
Ask Britannica AI about this topic
Britannica doesn't have a full article on this topic, so we've curated a list of our relevant coverage.
motion-picture technologyFilmThe ASA (American Standards Association) scale is an arbitrary rating of film speed; that is, the sensitivity of the film to light. If everything else is kept constant, the required exposure time is inversely proportional to the ASA rating. Negative films designed for original picture exposure are…
Read MoreThe ASA (American Standards Association) scale is an arbitrary rating of film speed; that is, the sensitivity of the film to light. If everything else is kept constant, the required exposure time is inversely proportional to the ASA rating. Negative films designed for original picture exposure are…
Read More
technology of photographySensitometry and speed…with the still widely used ASA speed). The second half (24°) is logarithmic, increasing by 3° for every doubling of the speed (and matching the DIN speeds still used in parts of Europe). A film of 200/24° ISO is twice as fast (and for a given subject requires half as…
Read More…with the still widely used ASA speed). The second half (24°) is logarithmic, increasing by 3° for every doubling of the speed (and matching the DIN speeds still used in parts of Europe). A film of 200/24° ISO is twice as fast (and for a given subject requires half as…
Read More