Accommodation of Eye

Hold your science book in your hand and read a single line. Now lift your eyes and look out of the window at a cloud in the sky. The letters looked sharp and the cloud looks sharp, and you did nothing on purpose to make that happen. Yet the two objects are at very different distances from you.

The distance of an object from the eye is its object distance. For the letters in your hand it is short, about 30 cm. For the cloud it is hundreds of metres. Now think about what happens inside the eye. Whatever you look at, its image must form on the retina, the light sensitive layer at the back of the eyeball. The distance between the eye lens and the retina is the image distance. The eyeball does not grow longer or shorter as your gaze moves, so this distance stays the same. The object distance changes a great deal, while the image distance does not change at all.

Think of a convex glass lens in the laboratory. When a candle is moved closer to it, the sharp image moves farther away from the lens, and you have to slide the screen back to catch it. With a glass lens, the image distance changes whenever the object distance changes. The eye cannot do this. Its screen is the retina, and the retina is fixed in place.

So the eye solves the problem from the other side. It leaves the screen where it is and changes the lens instead. The eye lens is soft and flexible, so its curvature can change. A lens with more curvature is thicker in the middle, bends light more strongly and has a shorter focal length. A lens with less curvature is thinner, bends light less and has a longer focal length. By changing its curvature, the eye lens makes the rays of light converge more or less, so that they always meet exactly on the retina.

The shape of the lens is changed by the ciliary muscles. These muscles form a ring around the eye lens and are joined to it by fine fibres called suspensory ligaments. When the ciliary muscles relax, the lens becomes thinner. When they contract, the lens becomes thicker.

Rays of light from a very distant object, such as a cloud, a mountain or the Moon, reach the eye almost parallel to one another. Parallel rays need only a little bending to meet at one point. This is what happens in the eye:

• The ciliary muscles relax.
• The lens becomes thin and its curvature decreases.
• The focal length of the lens increases.
• The nearly parallel rays are bent only a little and meet exactly on the retina.
• A clear image forms on the retina and the distant object is seen clearly.

Rays from a nearby object, such as the letters in a book, spread out from each point as they travel towards the eye. Spreading rays must be bent much more strongly before they can meet. This is what happens in the eye now:

• The ciliary muscles contract.
• The curvature of the lens increases and the lens becomes thicker.
• The focal length of the lens decreases.
• The rays from the nearby object are refracted more strongly and meet on the retina.
• A clear image forms on the retina and the nearby object is seen clearly.

The table sets the two cases side by side.

Accommodation goes on all day without you noticing it. When you copy a sum from the board into your exercise book, your eyes switch between the distant board and the nearby page again and again. Each time, the ciliary muscles relax or contract to match. A camera focuses by moving its lens forwards and backwards. The eye does the same job by changing the shape of a single lens.

Every eye has a limit to how far away and how close it can see clearly. The far limit comes first.

Infinity here means so far away that the rays arriving from the object are parallel. A normal eye sees the Moon and the stars clearly at night, and they are the most distant things you will ever look at. When the eye looks at an object at its far point, the ciliary muscles are fully relaxed, the lens is at its thinnest and its focal length is the greatest it can be.

Now try the other direction. Hold a pencil at arm's length and look at its tip. Keep your eyes on the tip and bring it slowly towards your nose. For a while the tip stays sharp, because the ciliary muscles contract more and more and the lens grows thicker and thicker. At a certain distance the ciliary muscles reach the greatest contraction they can manage. Bring the pencil any closer and the muscles cannot contract further, so the lens cannot become any thicker. The rays are no longer focused on the retina, and the tip looks blurred.

When the eye looks at an object placed at its near point, the eye lens has its maximum curvature. It is at its thickest and its focal length is the shortest. This is the opposite end from the far point, where the lens is at its thinnest and its focal length is the longest.

The near point is not the same for everyone. It can vary with a person's age and with the condition of the eye. The value of 25 cm is the one taken for a normal eye.

Now put the two limits together. An object placed anywhere between the near point and the far point is seen clearly. This stretch of distance is the range of vision.

So a normal eye sees a book held 25 cm away, a friend on the far side of the classroom, a hill across the valley and the Moon at night, all clearly. It cannot see clearly a thing held 10 cm from the eye, because that is inside the near point.

In people who have visual defects, the near point and the far point are not where they are in a normal eye. Their range of vision is different from that of normal vision.