Object-image relationships in a reduced schematic eye have been considered for both direct and indirect ophthalmoscopy. Certain distorting effects of lateral and longitudinal magnification in ophthalmoscopy have been defined and compared. The model described allows first order evaluation of imagery for both axial and oblique viewing of the patient's retina. Techniques have been described which allow partial evaluation of the factors developed in the text by (1) apparent projection of a fixed reference after-image located on the observer's retina, and by (2) selectively bleaching known areas on the patient's retina.
Enoch JM, Goldberg MF. Lateral and Longitudinal Magnification in Direct and Indirect Ophthalmoscopy. Arch Ophthalmol. 1971;86(5):536-547. doi:10.1001/archopht.1971.01000010538010