1. Apparatus for obtaining spherocylinder measurement of an eye comprising: a primary emitter of a light beam having one emitter pupil conjugate lens and one relay lens to form a secondary retinal light source from said light beam on said eye's retina; a photodetector comprising a light detector for measurement of at least a first retinal image and a second retinal image of said secondary retinal light source as observed by said photodetector respectively at distal and proximal positions relative to said secondary retinal light source and generation of signals proportional to the measured images, a field lens; and computation means responsive to said signals for calculating said spherocylinder measurement of said eye; wherein the field lens is positioned in a path taken by light traveling from the second retinal light source to the distal position of the photodetector, the field lens having a location and power so that as refractive error in the eye varies, the signals produced from the distal and proximal positions of the photodetector change with a linear relationship to one another.