Subjective refraction techniques in the frame of the three-dimensional dioptric space

A novel heuristic approach to the well-known representation of the dioptric power in a three-dimensional space is presented. It is shown how this theo retical framework is ideal for discussing the principles of several subject ive refraction methods. In particular, this formalism is used to justify th e stenopaic slit refraction, the Barnes subjective refraction technique, an d the Jackson cross-cylinder procedure. In view of this analysis, some modi fications to the traditional procedures are proposed.