  Abstract: Visual-angle has been used as the conventional unit to determine
the field-of-view (FOV) in traditional fundus photography. Recently emerging
usage of eye-angle as the unit in wide field fundus photography creates
confusions about FOV interpretation in instrumentation design and clinical
application. This study is to systematically derive the relationship between
the visual-angle {\theta}v and eye-angle {\theta}e, and thus to enable reliable
determination 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 quantitatively
evaluated. Systematical analysis indicates that reliable conversion between the
{\theta}v and {\theta}e requires determined nodal point and spherical radius of
the eye; and the conversion ratio is not linear from the central field to
peripheral region. Based on the eye model with average parameters, both angular
conversion ({\Delta}{\theta}e/{\Delta}{\theta}v) and retinal conversion
({\Delta}d/{\Delta}{\theta}v) ratios are observed to have a 1.51-fold
difference 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 for
reliable assessment of imaging systems with variable FOV.
