Biomedical Engineering Reference
In-Depth Information
Less defocused
Retinal blur circles
More defocused
Figure 11-2.
As the amount of defocus increases, the diameter of the retinal blur circle
also increases.
condition. In uncorrected (or undercorrected) ametropia and presbyopia, the small
pupil may improve visual acuity through the so-called pinhole effect.
While the smaller pupil may reduce the diameter of the blur circles, it also reduces
the amount of light that reaches the retina. A reduction in retinal illumination may
cause a reduction in visual acuity (for physiological, not optical reasons) that offsets
Large pupil
Retinal blur circles
Smaller pupil
Figure 11-3.
Decreasing the pupil diameter reduces the retinal blur circle diame-
ter. The smaller pupil blocks peripheral light rays from reaching the retina, thereby
decreasing the size of the blur circle.
