Human Eye
- Near point (D): 25 cm (minimum distance for clear vision — normal adult).
- Far point: Infinity (maximum distance for clear vision — normal adult).
- Accommodation: The eye changes focal length of its lens by changing curvature (ciliary muscles).
Eye Defects and Corrections
| Defect | Cause | Correction | Lens Used |
|---|---|---|---|
| Myopia (Short-sight) | Image forms in front of retina (eyeball too long or lens too curved) | Concave lens (diverging) | P < 0 |
| Hypermetropia (Long-sight) | Image forms behind retina (eyeball too short or lens too flat) | Convex lens (converging) | P > 0 |
| Presbyopia | Loss of accommodation with age | Bifocal lenses | Both P > 0 and P < 0 |
| Astigmatism | Non-spherical cornea | Cylindrical lenses | Special |
Formula for myopia correction: P = −1/d (d = far point in metres) Formula for hypermetropia: use lens formula with u = −25 cm (near point of normal eye), v = −near point of patient.