Stereoacuity and depth perception decrease with increased instrument magnification: comparing a non-magnified system with lens loupes and a surgical microscope.

PURPOSE: To evaluate the effect of instrument magnification used in eye surgery on stereoacuity and depth perception.
METHODS: Twenty-one subjects (10 clinical ophthalmologists familiar with loupes and operating microscopes and 11 non-ophthalmologists) with normal near vision and stereoacuity were tested with the Randot Stereotest viewed unmagnified, with a 4x loupes (450 mm focal length), and with a 16x operating microscope.
RESULTS: Total scores: 8 errors in 210 test steps with the unmagnified observations, 25/210 with loupes, and 30/210 with the microscope. The statistical differences in these scores were "statistically highly significant" for all three tests (p = 0.002); and "significant" for the unmagnified versus loupe (p = 0.007) and unmagnified versus microscope (p = 0.002). Test viewing through the microscope, the greatest errors occurred (total errors = 1840 seconds of arc), less with the loupes (total 1150") and least without magnification (total 220"). Errors and scores for 10 experienced ophthalmologists were no different (p > or = 0.55, p = 1.00) from the 11 non-ophthalmologist subjects.
CONCLUSIONS: Stereoacuity and depth perception decrease when viewing a test target with loupes or with a microscope, with the effect worsening as magnification increases. Familiarity with the magnifying equipment did not improve stereoacuity.