PURPOSE. In a perfect optical system, the smallest shift from the point of optimum focus results in blur. The clarity of the image in the human eye is not appreciated at a single point but over a range of distances, the dioptric equivalent of which is known as the depth of focus (DOF). This study aims at comparing the DOF between emmetropes and corrected ametropes in order to determine the effect of refractive error on DOF. METHOD. The DOF was measured for two groups- the first group consisting of 3 emmetropes and the second group consisting of 6 corrected ametropes (4 spherical and 2 astigmatic) in the age range of 20- 35 years. The spherical refractive error ranged from +2.00D to-5.00D with two subjects having an average astigmatism of 2.50D. A modified Badal optometer system was used to measure DOF. Subjects viewed a 20/30 target (92% contrast), under cycloplegia, through a 3mm artificial pupil. Two different subjective criteria were employed- one in which the subject reports smallest decrease in clarity, the other in which the Snellen optotype becomes totally illegible. The procedure was repeated for medium (60%) and low (27%) contrast levels.
RESULTS. In the first criterion, a significant decrease was observed in the ametropes (mean DOF=0.43D) as compared to the emmetropes (mean DOF=0.66D) with paired sample t-test for high (P=0.0064), medium (P=0.0356) and low (P=0.0025) contrast levels. No specific trend was observed with the second criterion. Analysis of DOF measurements between spherical and astigmatic ametropes did not reveal any significant difference for both criteria.
CONCLUSIONS. From the results of our pilot study, ametropes have a lower DOF as compared to emmetropes. This could be attributed to a difference in the concept of blur perception due to previous exposures to defocus in ametropes.