what is pinhole visual acuity ?
For individuals who do not have any type of ocular disease, a pinhole aperture can be a useful tool in determining if a refractive error is present or if a refractive change is needed.
The most useful pinhole diameter for clinical purposes is 1.2 millimeters. This size pinhole will be effective for refractive errors of +/– 5.00D.
A pinhole improves visual acuity by decreasing the size of the blur circle on the retina resulting in an improvement of the individual's visual acuity.
However, if the pinhole aperture is smaller than 1.2 millimeters, the blurring effects of diffraction around the edges of the aperture will actually increase the blur circle, causing the vision to be worse.
Individuals with macular disease, as well as other ocular diseases that affect central vision, may have similar or even reduced acuity when looking through a pinhole.
This is because the reduced amount of light entering through the pinhole makes the chart less clear.
Additionally, it can be difficult to use eccentric fixation through a pinhole. For this reason, individuals with ocular disease should not be told that a spectacle correction change will not improve their vision, based solely on their looking through a pinhole.
Careful retinoscopy along with a trial frame refraction is needed to determine whether an individual with pathology induced vision loss will benefit from a spectacle correction change.