field lens vs lens system

field lens and lens system both are nouns.

Word	Noun	Adjective	Verb	Adverb
field lens	Yes	No	No	No
lens system	Yes	No	No	No

As nouns, lens system is a hypernym of field lens; that is, lens system is a word with a broader meaning than field lens:

field lens: the lens that is farthest from the eye in an optical device with more than one lens
lens system: a transparent optical device used to converge or diverge transmitted light and to form images

Other hypernyms of field lens include lens, lense.

field lens (noun)	lens system (noun)
the lens that is farthest from the eye in an optical device with more than one lens	a transparent optical device used to converge or diverge transmitted light and to form images