The Human Eye

Accommodation is the ability of the human eye to adjust its focal length in order to focus clearly on objects at different distances.

How accommodation works

The crystalline lens in the eye is flexible. Its shape (and thus its focal length) can be changed by the ciliary muscles.

1. When looking at a near object:
   1. Ciliary muscles contract
   2. Suspensory ligaments loosen
   3. The lens becomes thicker
   4. This increases the refractive power of the lens
2. When looking at a distant object:
   1. Ciliary muscles relax
   2. Suspensory ligaments tighten
   3. The lens becomes thinner
   4. This decreases the refractive power of the lens

This process allows light rays from objects at different distances to be focused precisely on the retina, forming a clear image.

Limits of accommodation

There are two important points related to accommodation:

1. Near point (least distance of distinct vision):
   • The closest distance from the eye at which an object can be seen clearly.
   • For a normal eye, the near point is 25 cm.
2. Far point (maximum distance of distinct vision):
   • The farthest distance at which the eye can see objects clearly.
   • For a normal eye, the far point is at infinity.

Summary

Object Distance	Lens Action	Ciliary Muscle	Lens Shape	Focal Length
Near Object (≤25cm)	Lens thickens	Contracts	Thicker	Decreased
Distant Object (∞)	Lens thins	Relaxes	Thinner	Increased

Myopia or near-sightedness

1. This defect causes person to see near object clearly, while distant objects are not seen clearly.
2. The strength of the cornea and the eye lens combination is too great even when muscles of the eye are completely relaxed.
3. The focal length of the cornea and the eye – lens combination is always less than the distance to the retina.
4. Images of distant object are formed in front of the retina even when eye is very relaxed. However, an object that is closer can be brought into focus.
5. In this situation, the focal length of the cornea and the eye lens is so short that objects closer than the conventional (near point of 25cm) can be brought into focus. That is why this condition is called short sightedness (near sightedness).
6. Since the problem is that the strength of the eye – lens and the cornea combination is too great, the solution is to provide eyeglasses (or contract lenses) with negative lens.
7. The negative lens weakens the strength of the cornea and eye – lens just enough so that the resulting focal length when the eye muscles are relaxed matches the distance back to the retina so that distant images are now in focused.
8. The eyeglass lenses are negative lenses that means they are thinner in the middle than at the edges.

Hyperopia or far-sightedness

1. This defect causes a person to see distant objects only and short-distance objects are not seen clearly.
2. In the person with this condition, the strength of the cornea and the eye-lens combination is too weak when the eye muscles are very relaxed. Therefore, the image of a distant object is formed behind the retina.
3. The solution in the opposite of myopia. Victims should wear positive eye lenses, which strengthen the corner, and the eye lens just enough so that the resulting focal length when the eye is relaxed matches the distance to the back of the retina.

Astigmatism

1. This occurs when the focal length for the cornea and the eye's lens for an object oriented in some direction is not the same as for another located in a perpendicular direction.
2. The eye cannot bring the vertical and horizontal lines in a „+‟ symbol in sharp focus at the same time. (The axis of differing focal length need not be exactly horizontal and vertical).
3. The problem is that the cornea of the eye lens is not symmetrical. The solution is to use eyeglasses whose lenses are not symmetrical in a complementary way.
4. The cylindrical lens may be combined with additional positive or negative lenses.

Decreased accommodation

1. This condition typically occurs in middle-aged people.
2. The eye muscles gradually weaken with age, so that the range or accommodation is decreased.
3. People with this condition cannot bring near both objects and far objects into focus.
4. The weakening of the eye muscles often causes the focal length of the eye lens to increase as well so that many people of middle age tend to become far sighted.
5. Since the problem is adequate accommodation, no single lens can correct it and people with this problem usual needs bifocals.
6. Bifocals are glasses with two different lens strengths, one for near and one for distant objects.
7. The usual arrangement is that the bottom half of the lens is the near strength and the top half is the far strength.

Myopia is common name for impaired vision in which a person sees near objects clearly, while distant objects appear blurred. In such a defective eye, the image of a distant object is formed in front of the retina and not at the retina itself. Consequently, a nearsighted person cannot focus clearly on an object farther away than the far point for the defective eye.

This defect arises because the power of the eye is too great due to the decrease in focal length of the crystalline lens. This may arise due to either

1. Excessive curvature of the cornea, or
2. Elongation of the eyeball.

Correction: This defect can be corrected by using a concave (diverging) lens. A concave lens of appropriate power or focal length is able to bring the image of the object back on the retina itself.

Farsightedness, also called hypermetropia, common name for a defect in vision in which a person sees near objects with blurred vision, while distant objects appear in sharp focus. In this case, the image is formed behind the retina.

This defect arises because either

1. the focal length of the eye lens is too great, or
2. the eyeball becomes too short, so that light rays from the nearby object, say at point N, cannot be brought to focus on the retina to give a distinct image.

Correction: This defect can be corrected by using a convex(converging) lens of appropriate focal length. When the object is at N', the eye exerts its maximum power of accommodation. Eyeglasses with converging lenses supply the additional focusing power required for forming the image on the retina.

The camera

1. The eye and the camera has a have a convex lens which form a real and inverted image of an object.
2. The eye and the camera are blackened inside to prevent internal reflection. Rays of light which are not received on the retina or camera film are absorbed by the choroid layer of the eye or the black surface inside the camera.
3. The eye can regulate the amount of light that passes through the crystalline lens by using pupil while in a camera the diaphragm regulates light.
4. In the eye the image is formed in the retina while in the camera the image is formed on the photographic plate.
5. The eye can change the focal length of its lens by the contraction and relaxation of the ciliary muscles. In this way the eye can focus objects at different distance.