5

Retinoscopy of a model eye

The Refraction Certificate Examination may require you to complete objective refraction (retinoscopy) of a model eye within 5 minutes.

The model eye is on a simple stand and, fortunately, has been made to provide a scope reflex that is easier to interpret than real reflexes. However, since no trial frame can be used here, you need to take care when judging the working distance and in being aware of the cylindrical axis. This situation is similar to performing retinoscopy on children (who are averse to trial frames).

See Chapter 4 to learn how to use the retinoscope.

If working in plus cyls, refract the model eye using spheres until the least with movement is neutralised and leave a residual with movement in the perpendicular axis. Then place a plus cyl lens in front of the sphere (holding both lenses flush together) and rotate the cyl axis line so that it is orientated parallel to your scope slit. Continue to refract in this meridian until neutralised.

Great care must now be taken when recording your results. An approximation of the cyl axis must be made, since there is no trial frame to aid your recording of the cyl axis.

Furthermore, even if your working distance is typically 66 cm for refracting adults in trial frames, you will probably find that your working distance for refracting children without trial frames (and, therefore, model eyes) is reduced to 50 cm. If your working distance is 50 cm, it is necessary to add –2.00 sphere to your prescription to correct for working distance (rather than the –1.50 sphere that is added for a working distance of 66 cm).

Remember to state your working distance and its correction for the examiners. For example, if the retinoscopy gives –3.50/+1.75 @ 130, record your result as –5.50/+1.75 @ 130, corrected for a working distance of 50 cm.