The influence of letter size on the focusing response of the eye

The monocular and binocular steady state accommodation responses in 10 normal subjects were measured for different letter sizes presented at 6m and 0.4m observation distances. A computer-aided Badal laser speckle optometer system was used to measure accommodative responses. For each subject, and group-averaged data, accommodative responses were independent of letter limb subtense. Binocular and monocular responses averaged across subjects did not differ significantly. The magnitude of accommodative responses was related to the resting focus of accommodation and was independent of subjects' accommodative amplitude, refractive error and pupil size. The implications of these findings for clinical and human factor methods for examining visual performance are discussed.