Understanding the relationship between visual-angle and eye-angle for reliable determination of the field-of-view in...

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 confusion about FOV interpretation in instrumentation design and clinical application. This study aims to systematically derive the relationship between the visual-angle θ_v and eye-angle θ_e, and thus to enable reliable determination of the FOV in wide field fundus photography. FOV conversion ratio θ_e/θ_v, angular conversion ratio Δθ_e/Δθ_v, retinal conversion ratio Δd/Δθ_v, retinal distance and area are quantitatively evaluated. Systematic analysis indicates that reliable conversion between the θ_v and θ_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 (Δθ_e/Δθ_v) and retinal conversion (Δd/Δθ_v) ratios are observed to have a 1.51-fold difference at the central field and far peripheral region. A conversion table, including θ_e/θ_v, Δθ_e/Δθ_v, Δd/Δθ_v, retinal area and percentage ratio, is created for reliable assessment of imaging systems with variable FOV.