Power of Accommodation of the Eye

The human eye with its various parts and the roles which it plays are very important in our lives. Here, we are going to discover some of these roles, one of them is that it helps us to see. This may generally seem to us as a very simple process to just say it like that, but a series of operations occur from the point of light rays which is entering our eyes till we perceive the image of whatever we are seeing. One of these processes which we will see here is the power of accommodation of the eye.

What is Power of Accommodation of Eye?

The process by which certain muscles which are called ciliary muscles function to change the focal length of the eyes so that the image is clearly formed on the retina is known as the accommodation of the eye. This will vary for near objects and the ones which are distant and also for objects which are moving away or towards the eye. By adjusting the focal length, the eye is actually changing its lens power as well. So we can say that this is known as the accommodating power of the eye. The human eye has the power to change its accommodation.

We will now understand this with an example.

Keep a finger in front of you. Now you can see our finger try to focus only on the finger. 

You will notice that objects which are kept in the background tend to get blurry. 

Now we need to do the opposite. 

That is we need to keep our finger in front of our face but focus on something in the background. What happens now? Our finger is now seen as blurred.

So how does this accommodating power work? Considering varying distances we can ask this question.

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The answer is that if the object which we have kept for the experiment in consideration is at a distance for the image to form at the retina, that is,  the focal length has to be large. Here we will see that the ciliary muscles generally tend to relax thereby thinning the eye lens. The focal length which we are aware of will increase and the image is formed perfectly on the retina. 

Similarly, we can say that in the case of near lying objects the ciliary muscles contract and thereby thicken the lens. This causes a reduction in the focal length for ideal image formation.

Theories in Power of Accommodation 

• In 1855 the theory of Helmholtz — was the most widely held theory. It was proposed by Hermann von Helmholtz. It stated that while viewing a faraway object, the circularly arranged ciliary muscles relax allowing the lens zonules and suspensory ligaments to pull on the lens flattening it as well. The source which we are talking about is the tension and the pressure that the vitreous and aqueous humour exert outwards onto the sclera. On the other hand, while viewing a near object, the ciliary muscles contract, resisting the outward pressure on the sclera which causes the lens zonules to slacken which allows the lens to spring back into a thicker more convex form.

• In 1992 the theory of Schachar Ronald proposed what has been a bizarre geometric theory. It claims that the focus of the human lens is said to be associated with increased tension on the lens via the equatorial zonules.  When the ciliary muscles contract then the equatorial zonular tension is increased causing the central surfaces of the crystalline lens in nature to steepen the central thickness of the lens to increase and its anterior-posterior diameter and the peripheral surfaces of the lens to flatten. The tension on equatorial zonules is said to be increased during the accommodation process that is the anterior and posterior zonules are simultaneously relaxing.