Purpose: To investigate the effect of a Fresnel prism on contrast sensitivity and visual acuity values.
Methods: We measured contrast sensitivity and visual acuity in 19 healthy subjects after they had worn Fresnel prisms ranging from 5 to 40⊿, in 5⊿ steps. Contrast sensitivity and visual acuity values were converted to log contrast sensitivity values and logMAR values, respectively.
Results: The log contrast sensitivity values in each spatial frequency area (1.5, 3, 6, 12 and 18 c/d) and the logMAR of visual acuity both gradually decreased when prisms of 5 to 40⊿ were worn. Log contrast sensitivity decreased with increasing power of the prism; significant changes were obtained at each spatial frequency area: 0.66 at 35⊿, 0.69 at 20⊿, 0.49 at 15⊿, 0.34 at 15⊿ and 0.17 at 10⊿. Furthermore, the logMAR value at 10⊿, at which a significant decrease was seen, was 0.1.
Conclusions: These optical factors of low resolution are complex and controversial issues. However, the Fresnel prism proved to be more useful in orthoptics and low vision as these optical factors came to be understood.
Purpose: We investigated the effect of astigmatism and pupil size on visual function.
Methods: Included in this study were 11 eyes of 11 normal subjects. We used a modified CAT-2000 (Menicon, Aichi), with an electronic pupillometer built into the contrast visual acuity measuring device. Because control of pupil size was not possible, we used artificial pupils. Visual acuity with artificial pupil (2.0～5.0 mm, 0.5 mm step) and astigmatism (0～3.0 D, 0.5 D step) after cyclopentolate (Cyplegin®) instillation were measured by 100% contrast photopic vision.
Results: Visual acuity decreased significantly, pupil size becoming large at astigmatism over 1.0 D. Visual acuity decreased as astigmatism and pupil increased.
Conclusions: This study suggests that in the astigmatic eye, dilated pupil size diminishes retinal image quality by enlarging the circle of least confusion, affecting visual function.