The process of adjusting the eye’s focal length is called accommodation
The image must fall precisely on the retina to produce clear vision—that is, the image distance must equal the lens-to-retina distance.
- Because the lens-to-retina distance does not change, the image distance must be the same for objects at all distances.
- The eye manages this by varying the power (and focal length) of the lens to accommodate objects at various distances.
The accommodation reflex (or near response) is a three-part reflex that brings near objects into focus through lens thickening, pupillary constriction, and inward rotation of the eyes—eye convergence. The ciliary bodies anchor suspensory ligaments, collectively called zonule, which stretch the lens and alter its refractive power.
So how does this accommodating power work considering varying distances?
- Accommodation for near objects occurs from relaxation of the zonule.
- During far vision, the ciliary bodies relax, the zonule stretches, and the lens flattens.
- During near accommodation, the ciliary bodies contract (i.e., shorten), which relaxes the zonule and rounds the lens (i.e., thickens it). This brings the near object into focus.
You can actually put this power of your eye to test. Keep your finger in front of you and try to focus only on the finger. You will notice that objects in the background tend to get blurry. Now do the opposite. Keep your finger in front of your face but focus on something in the background. What happens now? Your finger is blurred.
Power of Accommodation
- The process by which certain muscles (called ciliary muscles) function, to change the focal length of the eyes so that the image is clearly formed on the retina is called the accommodation of the eye.
- This will vary for near and distant objects and also for objects moving away or towards the eye.
- The eye’s power to change its power by adjusting the focal length is called the accommodating power of the eye.
- The Power of accommodation for a person with normal eyesight is around 4 dioptre (unit of lens power).