Purpose To develop a technique based on visual reaction times that can evaluate the depth of focus in uncorrected vision or when using different modalities of simultaneous vision correction for presbyopia. Reaction Times (RTs) are known to increase in a predictable way as stimuli become less visible, i.e. as contrast decreases. Contrast reduction, produced by different amounts of defocus blur, was calculated by increased RT.
Methods RTs were measured to the onset of a sinusoidal grating with a spatial frequency of 4 c/deg and a contrast of 10%, displayed on a Sony F‐520 CRT monitor (mean luminance 30 cd/m2), using a VSG2/5 stimulus generator card. All measurements were performed monocularly (dominant eye) and binocularly at 3m viewing distance, with best spectacle sphero‐cylindrical correction for distance and natural pupils. RTs were measured for different degrees of blur, induced using positive powered lenses up to +4.50 D.
Results RT increased with defocus blur, with the effect being more pronounced for monocular compared to binocular viewing conditions. Depth of acceptable perceptual blur (depth of focus) was calculated from the second order polynomials, which fitted the RT vs. defocus blur functions. More specifically, the width of the polynomial fit was estimated for a specific RT. Regardless of the criterion chosen, the depth of acceptable blur was greater under binocular to monocular viewing.
Conclusion The technique described here offers a behavioural measure of the depth of focus, and allows evaluation under both monocular / binocular viewing conditions.