Hello A.  Vitreous is attached to retina at many places so difficult to detect angle of refraction B.  There is not a single ray of light but million of light rays falling at different angle... Moreover all points on retina is not equally sensitive to light.. Macula is most sensitive area and we are mostly concerned about macula B.  We have many instruments which give us fair idea about refractive status... C.  Refractive status of eye depends on many variables like size of eyeball, shape of cornea, shape, position and refractive index of lens etc.... So a single variable of angle of refraction  in my opinion will not serve the purpose