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This question already has an answer here:

I have heard of lenses being described as fast that have f/stop values of f1.2 and f/2.8.

My question is why are such low f/stop lenses called fast?

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I don't think this is a dupe. The focus here is on the "why." –  Paul Cezanne Aug 28 '13 at 10:21
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marked as duplicate by MikeW, mattdm, Michael Clark, AJ Henderson, Paul Cezanne Aug 30 '13 at 13:27

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

2 Answers

up vote 2 down vote accepted

Directly from the wikipedia, I couldn't have said it better myself.

A lens with a larger maximum aperture (that is, a smaller minimum f-number) is called a "fast lens" because it delivers more light intensity (illuminance) to the focal plane, achieving the same exposure with a faster shutter speed.

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To explain the why "low f/stop" part: this is a ratio. Hence the smaller the number, the bigger the amount of light.

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