<P> A pinhole occluder is an opaque disk with one or more small holes through it , used by ophthalmologists , orthoptists and optometrists to test visual acuity . The occluder is a simple way to focus light , as in a pinhole camera , temporarily removing the effects of refractive errors such as myopia . Because light passes only through the center of the eye 's lens , defects in the shape of the lens ( errors of refraction ) have no effect while the occluder is used . In this way the ophthalmologist , orthoptist or optometrist can estimate the maximum improvement in a patient 's vision that can be attained by lenses to correct errors of refraction . This can be used to distinguish visual defects caused by refractive error , which improve when the occluder is used , from other problems , which do not . The pinhole occluder can also be used in testing visual acuity in mydriatic patients . In this case the pinhole occluder compensates for the inability to contract the iris assisting the eye in obtaining a retinal projection similar to that of a non-cycloplegic eye . </P>
what is the effect of the pinhole on visual acuity testing
simple way to focus light