Pinhole Testing: The pinhole testing device can determine if a problem with acuity is the result of refractive error (and thus correctable with glasses) or due to another process. The pinholes only allow the passage of light which is perpendicular to the lens, and thus does not need to be bent prior to being focused onto the retina. The patient is instructed to view the Snellen chart with the pinholes up (below right) and then again with them in the down position (below left). If the deficit corrects with the pinholes in place, the acuity issue is related to a refractive problem.