<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> <P> Squinting and looking through a tiny hole made with a finger works similarly to a pinhole occluder, by blocking light through the outer parts of the eye's lens . An improvised pinhole has a similar but better effect . The same principle has also been applied as an alternative to corrective lenses: a screen of pinholes is mounted on an eyeglass frame and worn as pinhole glasses . </P>

What is the effect of the pinhole on visual acuity testing