field glasses vs opera glasses

field glasses and opera glasses both are nouns.

Word	Noun	Adjective	Verb	Adverb
field glasses	Yes	No	No	No
opera glasses	Yes	No	No	No

As nouns, field glasses and opera glasses are synonyms defined as:

field glasses and opera glasses: (plural) an optical instrument designed for simultaneous use by both eyes

Other synonyms of field glasses include binoculars.

field glasses (noun)	opera glasses (noun)
(plural) an optical instrument designed for simultaneous use by both eyes	(plural) an optical instrument designed for simultaneous use by both eyes