focal ratio vs f number

focal ratio and f number both are nouns.

Word	Noun	Adjective	Verb	Adverb
focal ratio	Yes	No	No	No
f number	Yes	No	No	No

As nouns, focal ratio and f number are synonyms defined as:

focal ratio and f number: the ratio of the focal length to the diameter of a (camera) lens system

Other synonyms of focal ratio include speed, stop number.

focal ratio (noun)	f number (noun)
the ratio of the focal length to the diameter of a (camera) lens system	the ratio of the focal length to the diameter of a (camera) lens system