VISION: OPTICS

D. Normal Refraction and Refraction Pathophysiology

1. Definitions

a. far point: the furthest distance at which a clear image can be formed on the retina; involves no accommodation

b. near point: the closest distance at which a clear image can be formed on the retina; involves maximum accommodation

2. Emmetropia: With no accommodation (ciliary muscles relaxed; minimum optical power), images from distant objects come to a focus on the retina. This is the "normal" condition. Far point is infinity (distant objects) and near point depends on maximum power of accommodation (about 20 cm for a 40 yo adult).

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3. Myopia (near sighted): With the ciliary muscles relaxed, images from distant objects come to a focus in front of the retina, and only nearby objects are focused clearly. Far and near points are closer than normal.

Correction:  negative lens (diverging) or contact lens with less curved surface (compared to cornea)

Correction:  radial keratotomy (surgical slits in peripheral cornea to flatten central cornea);  also corneal shaping with lasers

4. Hyperopia (far sighted): With the ciliary muscles relaxed, images from distant objects come to a focus behind the retina; some or all of accommodation power must be used to see distant objects clearly and, with severe hyperopia, even maximum accommodation may be insufficient. Near point further than normal and far point infinity, at best.

Correction: positive lens (converging) or contact lens with more curved surface (compared to cornea)

5. Presbyopia: Reduction in maximum accommodation of lens. Characteristic of aging. Near point becomes further away.

Correction: bifocals or trifocals (lenses with amounts of correction depending on viewing direction)

6. Astigmatism: Differing refractive power at different orientation angles (e.g. differing horizontal and vertical power). Result: there is no distance at which an image is in focus at all angles.

Correction: Asymmetric lens, with different refractive power at different angles