f number vs focal ratio

f number and focal ratio both are nouns.

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

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

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

Other synonyms of f number include speed, stop number.

f number (noun)	focal ratio (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