A new fundus camera technique to help calculate eye-camera magnification: a rapid means to measure disc size.

Objective: To find a simple means for calculating eye-camera magnification to permit estimation of true retinal object size from a retinal photograph.

Methods: The position of the focusing knob on 3 different retinal cameras (TRC-50F and TRC-50X; Topcon America Corp, Paramus, NJ; and the CR6-45NM Non-mydriatic Retinal Camera; Canon Inc, Tokyo, Japan) was measured during optic nerve photography and correlated with the refractive error, or spectacle refraction, of the subject (N = 11 for each camera).

Results: A strong correlation was found between focusing knob position and spectacle refraction for each of the 3 cameras tested (r = 0.96, r = 0.99, and r = 0.97, respectively).

Conclusions: The focusing knob position reflects the spectacle refraction of the eye being photographed, and spectacle refraction is known to correlate well with eye-camera magnification. Therefore, focusing knob position can be used to help calculate eye-camera magnification and, hence, true retinal object size.

Clinical Relevance: The true size of the optic nerve head is important for the diagnosis of glaucoma from a retinal photograph. This technique is a simple means to calculate optic nerve head size, which may be especially useful in mass retinal photographic screening programs.