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.


CONCLUSIONS
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.
Department of Ophthalmology, McGill University, Montreal, Quebec, Canada. quigley.wilson@sympatico.ca
Michael G
MG
Quigley
Pascale
P
Dube
D000886:Anthropometry / Q000379:methods; D006801:Humans; D009898:Optic Disk / Q000033:anatomy & histology*; D009900:Optic Nerve / Q000033:anatomy & histology; D010781:Photography / Q000295:instrumentation / Q000379:methods*