Pupil Size and Its Effect on Visual Resolution
The size of the blur circle on the retina generally increases as the size of the pupil increases. If a pinhole aperture is placed immediately in front of an eye, it acts as an artificial pupil, and the size of the blur circle is reduced correspondingly (Fig 3-4).
Figure 3-4 Light rays from each point on an object (upright arrow) form a blur circle on the retina of a myopic eye. The retinal image is the composite of all blur circles, the size of each being proportional to the diameter of the pupil (A) and the amount of defocus. If a pinhole is held in front of the eye, the size of each blur circle is decreased; as a result, the overall retinal image is sharpened (B).
(Courtesy of Neal H. Atebara, MD. Revised illustration based on a drawing by C. H. Wooley.)
The pinhole is used clinically to measure pinhole visual acuity. If visual acuity improves when measured through a pinhole aperture, a refractive error is usually present. The most useful pinhole diameter for general clinical purposes (refractive errors between −5.00 D and +5.00 D) is 1.2 mm. If the pinhole aperture is made smaller, the blurring effects of diffraction around the edges of the aperture overwhelm the image-sharpening effects of the smaller aperture. For refractive errors greater than 5.00 D, the clinician needs to use a lens that corrects most of the refractive error in addition to the pinhole.
After the best refractive correction has been determined, the pinhole can also be used with a dilated pupil. If visual acuity improves, optical irregularities, such as corneal and lenticular light scattering or irregular astigmatism, are likely to be present, given that the pinhole serves to restrict light to a relatively small area of the eye’s optics. (This technique also can be used to identify optical causes of monocular diplopia.) If visual acuity worsens, macular disease must be considered, as a diseased macula is often unable to adapt to the reduced amount of light entering through the pinhole.
Because of the refractive effects of the cornea, the entrance pupil is about 13%–15% larger than the actual pupil and displaced somewhat anterior to the plane of the iris. This explains why the anterior chamber is actually deeper than it looks.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.