Video-photorefraction: the relationship between blur circle diameter and refractive error.

When using a videorefractor (VPR-1) the margins of the defocused blur circles are sometimes poorly defined and difficult to locate. To investigate the effect this may have on the apparent refractive error, the relationship between blur circle diameter and refractive error was found by videoing circles of known size and inputting these data as pupil and blur circle diameters. The results indicate that for accuracy of 0.50 D blur circles need to be defined to approximately equal to 1 mm.