Clinical verification of the formula of Bennett et al. (1994) of determining the size of retinal features by fundus photography.

Purpose: To clinically verify the formula of Bennett et al. (Graefes Arch Clin Exp 1994; 232: 361) of determining the size of retinal features and to study the previously unknown conversion factor of Topcon fundus camera.

Methods: Fundus photographs were taken from 17 healthy volunteers with Topcon and telecentric Zeiss fundus cameras. The macula-disc centre distance was measured from Zeiss fundus photographs in metric units using the formula of Bennett et al. (Graefes Arch Clin Exp 1994; 232: 361). The conversion factor of Topcon fundus camera and the macula-disc centre distance in degrees were calculated. The latter was further used to calculate the theoretical location of the blind spot. The results of 12 participants were compared to the location of their physiological blind spot determined with visual field examination by octopus custom-made blind spot visual field program.

Results: The theoretical location of the blind spot correlated well with the corresponding location of the physiological blind spot in the visual field. The magnification of Topcon fundus camera was close to a constant, and thus, the previously unknown conversion factor could be determined.

Conclusions: The location of the physiological blind spot in the visual field can be derived from fundus photographs using the formula of Bennett et al. (Graefes Arch Clin Exp 1994; 232: 361), proving it to give a close approximation of the size of retinal features. Furthermore, the conversion factor of Topcon fundus camera was close to a constant, and thus, it can be considered to function close to telecentric design.