Image Formation and Accommodation

Introduction

The eye is a complex organ. It allows animals to view the world around them and the objects in it. Whether the objects are far or near, eyes allow us to see and place them. Eyes also allow us to see and process various shapes and colours. In some ways, the eye is like the camera of the human body. 

The human eye is a complex organ, and it is made up of many different parts that all have their specific functions. 

The human eye can broadly be divided into two parts to understand its composition: the outer part of the eye and the internal part of the eye. 

The outer part of the eye includes the cornea, the conjunctiva, the sclera, the iris, and the pupil. In other words, these are the parts of the eye that are exposed to the world. The internal part of the eye, on the other hand, includes the retina, the lens, the aqueous humour, and the optic nerve of the eye. 

Crystalline Lens of the Eye

The iris of the eye has a small pore called the pupil. The pupil allows light to pass through the eye and fall on the lens. 

Found at the back of the iris, the crystalline lens allows the eye to adjust to tiny focal lengths, allowing the brain to focus on specific objects depending on how near or far they are. The crystalline lens is essential for the eye’s focus, allowing the final true image to be formed on the eye’s retina.  

This adjustment of the eye’s lens to see a clear image of what it is looking at is scientifically known as the power of accommodation of the eye. 

Ciliary Muscles in the Eye

The lens of the eye is made up of a fibrous jelly-like substance known as ciliary muscles. Ciliary muscles allow the eye to change its curvature to see across distances. 

Ciliary muscles are circular muscles that tighten and relax zonules in the eye in order to change the shape of the lens. Zonules are the fibres that hold the lens of the eye in place and allow it to change shape during the process of accommodation or adjusting sight based on distance. 

Zonules and ciliary muscles work together to adjust the lens’s curvature by changing the lens’s focal length.

When an object is close to our eye, ciliary muscles contract, become smaller in size, and the curvature of the lens increases. This makes the lens thicker. The increased curvature of the lens results in a decrease in its focal length. This process of increasing the curvature of the lens and decreasing its focal length allows us to focus on and see a nearby object clearly.

When an object is far away, on the other hand, the ciliary muscles relax. The lens of the eye becomes less curved, and its focal length increases. This allows us to focus on and see objects at a distance clearly.

Power of Accommodation of the Eye

The power of accommodation of the eye refers to the ability of the eye’s crystalline lens to change its focal length by increasing or decreasing its curvature. The power of accommodation of the eye is mainly responsible for the eye’s ability to form a clear image of what it is seeing on the retina.

In other words, it is the ability of a lens to change its focal length to see far and near objects clearly. This process is called the power of accommodation.

Calculating the Power of Accommodation

The power of accommodation of the eye may be calculated using the following formula:

P=1f

Where P is the power of the eye, and f is the focal length 

For example: if an object is at a distance of 25cm, what is its power of accommodation?

Distance = 25cm. Therefore, the focal length, f,  is 0.25m. 

Using the formula P=1f,

P=10.25=4D

The eye’s lens is convex. Thus, its power will be taken as positive. Thus, the power is +4D.

Characteristics of Power of Accommodation

The power of accommodation for an individual with normal eyesight is approximately +4D.

The focal length of the eye lens has a minimum and maximum limit. As a result, it will not be reduced beyond a specific limit. This is observable by holding an object such as a printed page very close to the eye. Attempting to read it from such a close distance will strain the eye, and the image will appear blurred to you. 

Example of Power of Accommodation

If a finger is held close to the eye and sight is focussed on it, everything in the background will become blurred. Similarly, if the focus is maintained on the background, the finger will appear blurred to you.

A simple example such as this leads to the conclusion that the eye can focus on one object or one line of sight only at any given time. Attempts to focus on objects placed at multiple distances results in constantly shifting focus. This strains the eye and may even cause a headache. 

Human eyes need to maintain a minimum distance from the object being viewed to see it clearly. This minimum distance is known as the least distance of distinct vision. In other words, this is the closest point at which an object may be placed so it is seen clearly without straining the eyes of an averagely sighted human being. For adult human beings, this distance comes up to about 25 centimetres.

Similarly, the human eye cannot see beyond a given point or distance. This point is known as the farthest point of the eye’s vision. While the human eye can theoretically see infinite distances. However, the curved shape of the earth, or the earth’s curvature, limits just how far the human eye can see. For example, the moon is 384,400 km away from the earth, while the sun is 147.37 million km away. And yet we can see them both clearly while they are in our line of sight. The same logic applies to our ability to see stars as well.

As human beings age, their eyes slowly degenerate, and their eyesight becomes weaker. As a result, they cannot see very clearly. The lenses of their eyes often develop milky, cloudy impurities known as cataracts. While cataracts may cause partial or even complete vision loss, they may be easily corrected through cataract surgery.

Helmholtz Theory of Power of Accommodation

This widely accepted theory of the power of accommodation was proposed by a German physicist known as Hermann von Helmholtz in 1856.

He stated that “when viewing a distant object, the circularly arranged ciliary muscle is relaxed, allowing the lens zonules and suspensory ligaments to pull on the lens, flattening it in the periphery. The source of the tension is the pressure that the vitreous and aqueous humour exert outwards onto the sclera.” 

This means that when we view objects at far distances, ciliary muscles, which are arranged in a circular formation, become relaxed. This allows the lens to take on a thinner form. Similarly, the ciliary muscles contract while viewing nearby objects, allowing the lens to take on a thicker form.

Conclusion

The power of accommodation is the ability of a lens to adjust its focal length for the formation of a clear image on the retina. When an object is near the eye, the eye lens becomes thicker, its focal length decreases and ciliary muscles contract. When an object is at a distance from the eye, on the other hand, the ciliary muscles relax, and the lens becomes thin. Thus, its focal length increases. 

An average human eye has a minimum distance distinct vision of 25 cm and power of 4D. This power should be positive as the lens in the human eye is a double convex lens. 

The ciliary muscles are responsible for adjusting the power of accommodation of the eye lens.