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

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 and eye-angle θ, and thus to enable reliable determination of the FOV in wide field fundus photography. FOV conversion ratio θ/θ, angular conversion ratio Δθ/Δθ, retinal conversion ratio Δd/Δθ, retinal distance and area are quantitatively evaluated. Systematic analysis indicates that reliable conversion between the θ and θ 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/Δθ) and retinal conversion (Δd/Δθ) ratios are observed to have a 1.51-fold difference at the central field and far peripheral region. A conversion table, including θ/θ, Δθ/Δθ, Δd/Δθ, retinal area and percentage ratio, is created for reliable assessment of imaging systems with variable FOV.