Defects of vision and correction

Medical Physics | AQA A-Level Physics

MyopiaShort-sightedness. The eye focuses distant objects in front of the retina because the eyeball is too long or the lens is too powerful. Corrected with a diverging (concave) lens. (short-sightedness): distant objects are focused in front of the retina. The far point is closer than infinity. Corrected with a diverging (concave) lens that diverges light before it enters the eye, effectively moving the image back onto the retina.
HyperopiaLong-sightedness. The eye focuses near objects behind the retina because the eyeball is too short or the lens is too weak. Corrected with a converging (convex) lens. (long-sightedness): near objects are focused behind the retina. The near point is further than 25 cm. Corrected with a converging (convex) lens to add extra convergence.
AstigmatismA defect where the cornea or lens has different curvatures in different planes, causing blurred vision in one orientation. Corrected with a cylindrical lens.: the cornea or lens has different curvatures in different planes (e.g. horizontal vs vertical), so light is focused at different points depending on orientation. Corrected with a cylindrical lens that only refracts light in one plane.
A prescription for astigmatism uses three values:
- SPH (sphere): the power needed for the overall correction (myopia or hyperopia).
- CYL (cylinder): the additional power needed in the astigmatic plane.
- Axis: the angle (in degrees, 0 to 180) at which the cylindrical correction is applied.

Common Mistake HIGH

Students often: Mixing up which lens corrects which defect.
Instead: Myopia (short-sight) needs a diverging lens (to spread rays out). Hyperopia (long-sight) needs a converging lens (to bring rays together sooner). Think: the correction does the opposite of the problem.