The Osterberg Graph
Only 5% of photoreceptors in the retina are cones, and these are concentrated at the fovea. As reported by Osterberg in 1935, the density of cones drops off rapidly (but not abruptly) just several degrees from the fovea, and rods begin to predominate outside of the central 5 degrees or so. This known distribution of rods and cones is important in the interpretation of the PIL in SD OCTs. The width of the OCT section (6mm) is denoted on the Osterberg graph by the vertical dashed green lines.
Osterberg GA. Topography of the Layers of rods and cones in the human retina. Acta Ophthalog 1935