Contrast sensitivity and the Stiles--Crawford effect.

We investigated the influence of the Stiles-Crawford effect (SCE) of the first kind on the contrast sensitivity function using the apodisation model of the SCE. The SCE was measured for the right eyes of two subjects using an increment threshold technique involving a two-channel Maxwellian-viewing system. Filters made of photographic film neutralised or doubled the SCE. Contrast sensitivities were measured with a 6 mm pupil diameter, defocus to +/-2D, and three SCE conditions (normal, neutralised and doubled). Modulation transfer functions were derived after measuring transverse aberrations with a vernier alignment technique, and were used to predict contrast sensitivity functions (CSFs). The measured CSFs were, in general, reasonable matches with the predicted CSFs. In particular, both demonstrated definite undulations ("notches") as defocus level increased. The influences of the SCE-modifying filters were generally of similar magnitude and direction to predictions, thus supporting the apodisation model of the SCE. The magnitudes of SCE influence between SCE-neutralised and SCE-doubled conditions were usually small at about 0.2-0.3 log unit, with a maximum influence of 0.5 log unit. Influences of the SCE were greater for myopic than for hypermetropic defocus. As measured by the CSF and an apodisation model, this study is in agreement with previous theoretical work and one experimental study in indicating that the SCE plays a minor role in improving spatial visual performance.