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Visual-angle has been used as the conventional unit to determinethe field-of-view (FOV) in traditional fundus photography. Recently emergingusage of eye-angle as the unit in wide field fundus photography createsconfusions about FOV interpretation in instrumentation design and clinicalapplication. This study is to systematically derive the relationship betweenthe visual-angle {\theta}v and eye-angle {\theta}e, and thus to enable reliabledetermination of the FOV in wide field fundus photography. FOV conversion ratio{\theta}e/{\theta}v, angular conversion ratio{\Delta}{\theta}e/{\Delta}{\theta}v, retinal conversion ratio{\Delta}d/{\Delta}{\theta}v, retinal distance and area are quantitativelyevaluated. Systematical analysis indicates that reliable conversion between the{\theta}v and {\theta}e requires determined nodal point and spherical radius ofthe eye; and the conversion ratio is not linear from the central field toperipheral region. Based on the eye model with average parameters, both angularconversion ({\Delta}{\theta}e/{\Delta}{\theta}v) and retinal conversion({\Delta}d/{\Delta}{\theta}v) ratios are observed to have a 1.51-folddifference at the central field and far peripheral region. A conversion table,including {\theta}e/{\theta}v, {\Delta}{\theta}e/{\Delta}{\theta}v,{\Delta}d/{\Delta}{\theta}v, retinal area and percentage ratio, is created forreliable assessment of imaging systems with variable FOV.

biomedical optics express

Understanding the relationship between visual-angle and eye-angle for  reliable determination of the field-of-view in ultra-wide field fundus  photography