Eye Accommodation: How Our Eyes Focus

Eye accommodation is the natural process that allows you to keep your eyes focused on an object with excellent depth perception when that object — or you — are in motion. The process is made possile through a series of eye changes by its lenses.

Our eyes lose their accommodation ability as we get older, particularly as we approach our mid-forties. 

Other accommodation dysfunction problems are non-age-related, affecting children and adults. These conditions also impede the ability to focus on near or distant objects. 

What Is Eye Accommodation?

Eye accommodation is when the eyes adjust their optical power to maintain an object in focus in the retina at varying distances. The process is achieved primarily by the lens changing its shape when focusing shifts from distant to near object and vice versa.

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How Eye Accommodation Works

Accommodation works primarily by changes in the eye lens. Eyes adjust their lens to cause either more or less refraction of light so that the object is appropriately focussed on the retina.

A high curvature creates a shorter focal length for near objects, while a flatter curvature is used to create a longer focal length for images at a further distance.

Accommodation is a naturally occurring ability for people with normal vision. But accommodation changes are common as we age. This is especially true when we start to approach age 50.

How Eyes Change During Accommodation 

There are three main changes to the eye that happen during accommodation. The changes affect the pupils, lens and overall eyes. The happen from convergence to pupillary constriction to lens accommodation

Convergence 

Convergence is the inward movement of both eyes simultaneously when focusing on a nearby object. Its purpose is to keep the image focussed on the fovea, which is the part of the retina with the highest visual perception. The inward movement happens by the joint contraction of each eye’s muscles, known as medial recti. 

Convergence of the eyes facilitates binocular vision, which is the ability to keep the visual focus on an object which creates a single visual image. You need convergence to have proper depth perception and to see at relative distances. 

When your eyes focus on a nearby object, they move towards one another the closer the object is or gets.

When looking at objects in the distance, they move apart, adjusting to an appropriate field of view. 

Pupillary Constriction 

With pupillary constriction, also called miosis, the sphincter muscles of the iris contract, reducing the pupil’s diameter. In this constricted state, the pupil restricts the amount of light getting into the eye. 

Through pupil constriction, the eyes prevent divergent rays from hitting the retina’s periphery, which could cause a blurred image. Instead, by constricting, the pupil forces light to the center of the fovea, creating a sharper vision. It also increases the depth of vision.

Lens Accommodation 

Lens accommodation refers to how the lenses change their shape to focus on objects at different distances. Two opposing forces determine the lens’s shape:

• The lens’s internal elasticity, which keeps the lens curved
• The external force exerted by the zonule fibers, which flatten the lens when they are stretched

Zonule fibers are attached to the ciliary muscles. These muscles contract and relax, depending on how far away an object is.

• When viewing nearby objects (in an accommodation state): Ciliary muscles contract in a forward and inward movement. That relaxes the zonule fibers, allowing the internal elasticity of the lens to prevail, which increases the curvature of the lens and makes it thicker. As a result, the lens’s refractive power increases, allowing it to produce a sharper image on the retina. 
• When looking at distant objects: Ciliary muscle relax and place tension on the zonule fibers. The tension exceeds the lens’s internal elasticity. The lens stretches, making it flatter and thinner. This change decreases its refractive power to the appropriate value for a sharp image of a distant object. 

Why Accommodation Reflex Is Important 

The accommodation reflex is important because it enables you to focus clearly on objects that are up close when you are shifting from viewing objects far off. All the three eye changes must happen for you to see the near objects clearly.

For instance, if the eye convergence does not happen correctly, you will have double vision. On the other hand, if pupils stay dilated, light will not focus on the fovea, leaving the image blurred. 

And if the lenses do not change shape, then the eyes cannot focus on items at varying distances. The rounding and flattening of the lenses adjust the refractive power, allowing the proper focusing of light on the retina for clear vision. 

How Eye Accommodation Changes with Age 

With age, your eyes lose their ability to focus clearly and comfortably on up-close objects. The medical term for the condition is presbyopia, which usually takes place between the ages of 40 and 45 years. The main changes affect the lens and ciliary muscles. 

Age robs the lens of its internal elasticity, which affects its ability to curve and increase the refractive power needed to create and sharpen the focus of near items.

Another cause is the hardening and increased size of the sclerosis, the white of the eye. That makes it more difficult for the lens to curve the way it wants. 

Ciliary muscles also atrophy with age, leaving the lens stretched and flat, decreasing the eyes’ refractive power. 

Accommodative Dysfunction 

Accommodative dysfunction refers to a condition in which an adult or a child has difficulty with their eye-focusing system. The dysfunction results in blurred vision for up-close or distant objects, but often the problem is with near vision.

Accommodative dysfunction also refers to eye-focussing problems unrelated to aging. 

There are three types of accommodative dysfunction:

• Accommodative Insufficiency: You have trouble keeping the focus on near items
• Accommodative Infacility: You have trouble shifting focus between near and distant objects and back again
• Accommodative spasm: You experience a spasm in the focussing muscle, which hinders full relaxation of the muscle and causes blurred distant vision. 

Symptoms of accommodative dysfunction include:

• Blurred vision at near objects
• Eye fatigue and eye strain 
• Headaches 
• Difficulty in sustaining attention 
• Poor reading fluency 
• Blurred distant vision after doing near vision work
• General fatigue at the end of the day

Treatment for accommodative dysfunction typically involves prescription special lenses to combat the eye strain and a custom-made vision therapy program.

References

1. Accommodative Dysfunction. (2022). American Optometric Association.

2. Physiology, Accommodation. (July 2021). National Center for Biotechnology Information.

3. Accommodation For the Aging Eye (November 2016). Review of Ophthalmology.

4. Chapter 15 – Clinical Examination of the Cranial Nerves. (2015). Nerve and Nerve Injuries – Vol 1: History, Embryology, Anatomy, Imaging, and Diagnostic.

5. The Physiologic Mechanism of Accommodation. (April 2014). Cataract & Refractive Surgery Today Europe.