The Stiles-Crawford effect and subjective measurement of aberrations.

We considered the influence that the Stiles-Crawford effect (SCE) has on the measurement of subjective monochromatic and transverse aberration measurements. The SCE was measured with a two channel Maxwellian-viewing system. Transverse aberrations were measured using a vernier alignment technique in three subjects, with the natural SCE operating, with the SCE neutralised by filters optically conjugate with the eye's pupil, and for one subject with filters that shifted the SCE by more than 2 mm. As pupil diameter increased from 1 to 5 mm diameter, without the filters the slope of the transverse aberration versus position in the pupil decreased, e.g. for chromatic aberration this decreased by approximately 90%. The filters had little influence on transverse aberration. The results indicate that subjects do not use the centroid of the image of a blurred line target for alignment, but may rely very much on other cues.