Ocular structure, physiology and optical calculations
Ocular optics connects the refracting surfaces, axial dimensions, pupil, accommodation and the plane of correction. A simplified eye model is useful only when its assumptions are clear. Myopia and hyperopia describe the relationship between focus and retinal position, while astigmatism requires assessment of principal meridians. Plus-cylinder and minus-cylinder prescriptions can describe the same optical correction after proper transposition. Keep the sphere, cylinder sign and axis convention together rather than change one component in isolation.
Vergence and vertex-distance calculations require consistent units and sign conventions. For a thin spectacle lens with power Fs at distance d meters from the cornea, the equivalent contact-plane power in the simple model is Fc = Fs / (1 - d x Fs). A -5 D spectacle lens at 0.012 m corresponds to -5 / 1.06, approximately -4.72 D at the corneal plane. This is an optical equivalence exercise, not a complete contact-lens fitting prescription. Tear-lens effects, lens geometry and the actual eye can require further clinical assessment.
Prentice's rule gives prism magnitude as decentration in centimeters multiplied by the absolute lens power in diopters. Direction must be reasoned separately: a plus lens behaves as prisms with their bases toward the optical center, while a minus lens has bases directed away from it. Looking below the center of a plus lens therefore gives a different vertical base direction from looking below a minus lens. Do not confuse displacement of the lens center with the position of the gaze point relative to that center. A magnitude calculation can be correct while the directional answer is wrong.
The cornea, crystalline lens, aqueous and vitreous contribute differently to the optical system. Corneal transparency relates to organized stromal structure and hydration regulation; retinal phototransduction transforms light into cellular signaling rather than producing an action potential in each photoreceptor. Rod and cone distributions and convergence support different sensitivity and resolution characteristics. Acuity, contrast sensitivity, color vision, fields and stereopsis measure different aspects of visual function. A Snellen fraction can be converted into a minimum-angle or logMAR representation under the relevant chart convention, but an acuity result alone is not a localization or complete measure of everyday visual performance.