Why is a wide-aperture lens called a "fast" lens?
Asked 4/14/2012
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In photography, lenses with larger maximum apertures are often described as "fast," which can be confusing because lenses do not have a speed in the usual sense. Why did this term come to mean a lens that lets in more light?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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Indeed, Speed is way too overloaded in Photography, we have lens speed, shutter-speed and ISO speed!
These terms indirectly refer the possible shutter-speed. A fast lens has a bigger maximum aperture (represented by smaller numbers) which lets more light in. As a consequence, a fast lens lets you use fast shutter-speeds.
The same is true of ISO. A high ISO is called fast because you can use a faster shutter-speed with it.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
14y ago
0
Generated from our catalog & community — verify before relying on it.
"Fast lens" refers indirectly to shutter speed, not to the lens physically moving faster.
A lens with a larger maximum aperture, such as f/1.4 or f/2.8, lets in more light than a lens limited to f/4 or f/5.6. Because more light reaches the sensor or film, you can use a shorter exposure time for the same scene brightness and ISO. In other words, the lens allows a faster shutter speed.
That is why wide-aperture lenses came to be called "fast" lenses, while lenses with smaller maximum apertures are called "slow." The term is tied to the exposure relationship between aperture and shutter speed.
Photography uses "speed" in a few related ways, which adds to the confusion: shutter speed, lens speed, and ISO speed all describe settings that affect how quickly enough light can be captured for a proper exposure.
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