Purpose: To evaluate the magnification characteristics of an Optical Coherence Tomograph (STRATUS OCT 3000) used to measure optic disc size.
Methods: An eye model was designed, built and used to measure changes in the magnification of the Optical Coherence Tomograph (OCT) with variation in the axial length of the eye and OCT correction lenses. Theoretical calculations using ray-tracing were used to compare the experimental and theoretical results.
Results: OCT-performed fundus measurements have a magnification that depends on two factors: eye axial length and optical correction with the focusing dial of the OCT. Theoretical calculations showed that the size of the optic disc image for greater axial lengths of the eye model (myopic eye) was less than that for smaller axial lengths (hyperopic eye). The disc diameter measurements of the image obtained with the OCT did not agree with the real disc size. By focussing with the focusing dial of the OCT, the power of the optics can be varied from -12.0 D to +12.0 D and the image size is decreased for all axial lengths. The maximum variation of the disc area was 2.1%. Differences in area measured at intervals of 2.0 D after correcting the eye refraction for measurements with the OCT were equivalent to the repeatability error at that point.
Conclusions: Our results confirm that any variation in the optical system of the eye, in the OCT, and/or in the distance between them, induces a change in the magnification of the tomograph images. These variations will affect the measurements of any of the structures in the retinal plane.