The eye can be compared to a camera. The front section is composed of two natural lenses: the cornea, which is in front of the iris and the lens, which lies behind it.
The iris gives the eye its unique colour and acts as a diaphragm by controlling the amount of light that penetrates the eye through the pupil opening (black circle in the centre). The pupil contracts in bright light and dilates in dim light.
The cornea is a rounded window. Its curve gives it the ability to focus, similar to how a camera lens works.
The lens is transparent and flexible from birth. It is kept inside an envelope, or capsule, that is attached to the muscles.
When the muscles contract, the lens becomes rounder, like a magnifying glass. This action is called accommodation and it allows the eye to adjust its focus as an object moves closer. This action is like the zoom feature on a camera.
Light rays pass through cornea and lens to converge at a focal point. After penetrating the vitreous humor, which is a gelatinous substance, the point forms an image that is captured by the retina and projects it onto the back of the eye, similar to how camera film works. The image is then sent as nerve pulses to the brain by the optic nerve.
Astigmatism is caused when the curvature of the cornea or lens is arched in one axis and flatter in the opposite axis. Learn more about astigmatism here.
Astigmatism
Presbyopia is caused by the loss of natural accommodation of the lens that comes with age. Learn more about presbyopia here.
Presbyopia
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