VII.Optics of Retinoscopy

Rays coming out of Subject's Eye. (Reflex & Projection Stages)

If the illuminated patch on Patient's retina is away from the principal axis, the rays coming out will not enter Observer's eye. When the illumination is moved across the fundus towards the principal axis the rays coming out enters observer's eye.

VII.a) In Finding 1.The first ray of light entering observer's eye is from the same edge of the pupil as the first position of retinal illumination. As the illumination on the retina moves towards the principal axis the light reflex in the pupil also moves in the same direction. The last ray of light entering the observer's eye is from the other edge of the pupil. (see diagrams). This gives a with movement reflex in the pupil.( seen in emmetropia, hypermetropia and myopia less than 1D - observer at 1 metre from patient.)

VII.b) In Finding 2. The first ray of light entering observer's eye is from the opposite edge of the pupil when the position of the retinal illumination is considered. (Light rays cross at FP and the diverging rays are entering observer's eye). As the retinal illumination moves towards the principal axis the light reflex in the pupil moves in the opposite direction. The last ray of light entering the observer's eye is from the other edge of the pupil. (see diagrams). This gives an against movement reflex in the pupil. (seen in myopia more than 1D - observer at 1 metre)

VII.c) In Finding 3. As the retinal illumination moves towards the principal axis, the rays coming out will focus on the nodal point of observer's eye. This makes the pupil of the patient uniformly illuminated. (The rays coming out through all the parts of the pupil are focused at the nodal point of observer's eye). By a slight shift of retinal illumination this focal point is displaced away from observer's eye making patient's pupil uniformly dark. (see diagrams) This finding you get as end point of retinoscopy and in eyes with 1 D myopia. (observer at 1 metre from the patient.)

From the description and the diagrams you know the position of the FP in each finding. (behind your position or in front of you or at your nodal point). Now it is easy to decide which type of lens you have to use to reach the point of reversal. (convex lens if FP is behind the observer, ie in 'with movement' and concave lens if FP is in front, ie in 'against movement') Then by changing the power of lens you reach the end point.

Here movements are described in relation to movements of the retinal illumination. Not related to the movement of the mirror. They differ with type of mirror (plane or concave).

(Details discussed along with discussion of the illumination stage of retinoscopy.)
nodal point