Reduced visual acuity is the most notable feature of functional amblyopia--so much so that it is common to think that it is the only, main, or even the fundamental defect. To identify all amblyopic eyes by acuity alone would require an acuity criterion of 6/6 (20/20) or somewhat better acuity--a criterion that would necessarily include more normal than amblyopic eyes. The association of amblyopia with strabismus and anisometropia is sufficiently specific to make these two nonacuity features useful in identifying amblyopia. The diagnosis of amblyopia derives from a syndrome of features, most of which are nonacuity features. Two recently quantified nonacuity features--spatial uncertainty and distortion--are not only clinically useful but they may be the fundamental defects in strabismic amblyopia, with impaired resolution being fundamental in anisometropic amblyopia.