Compound eyes are found among the arthropods (insects and kin), and are composed of many simple facets which give a pixelated image (not multiple images as is often believed).
Diagram of a human eye. Note that not all eyes have the same anatomy as a human eye.
Light from a distant object and light from a near object being brought to a focus
In most vertebrates and some mollusks the eye works by projecting images onto a light-sensitive retina, where the light is detected and transmitted to the brain via the optic nerve. The eye is typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often a muscle called the iris that controls how much light enters.
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2 Parts of the eye
4 See also:
In order for light rays to be brought to a focus they must be refracted. The amount of refraction required depends on the distance of the object which is being viewed. A distant object will require less bending of light than a nearer one. Most of the refraction occurs at the cornea which has a fixed curvature. The remainder of the required refraction occurs at the lens. The lens can be pulled flatter or rounder by muscles, which adjust the power of the lens. As we age we lose this ability to adjust the focus. Such a condition is known as presbyopia.