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A New Fundus Camera Technique to Help Calculate Eye-Camera Magnification
A Rapid Means to Measure Disc Size
Michael G. Quigley, MD;
Pascale Dubé, MD
Arch Ophthalmol. 2003;121:707-709.
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.
From the Department of Ophthalmology, McGill University (Drs Quigley and Dubé), and the Department of Ophthalmology, University of Montreal (Dr Quigley), Montreal, Quebec. Dr Quigley has applied for a patent on the process described in this article.
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