The human eye is a marvelous design that employs many of the features of a camera. Most of the bending or focusing of the light occurs as the light initially passes through the cornea. Behind the cornea is the iris which controls the amount of light admitted. Behind the iris is the lens of the eye, also called the crystalline lens. The lens acts as a fine adjustment to the focus. It is pliable and can change its shape and focal length as the ciliary muscles attached to it exert forces on it.

The closest distance at which an object can be clearly seen or focused by the eye is called the near point. Likewise, the far point is the maximum distance at which an object can be clearly seen or focused. Nearsightedness or myopia means that nearby objects can be focused or seen clearly but distant objects can not be clearly focused; the far point is not infinity. Farsightedness or hyperopia means that distant objects can be focused or seen clearly but nearby objects can not be clearly focused.

Two or more lenses may be used in combination. In that case, the image of the first lens acts as the object for the next lens.

A converging lens may be used as a simple magnifier; it can produce an image at infinity that can be clearly focused by the eye, producing a larger image on the retina.

For a simple magnifier, the magnification is given by

M = 25cm / f

where f is the focal length of the lens.

A microscope is used for viewing small things; we usually mean a two-lens or when we say "microscope". The first or objective lens produces a real image that is then viewed with an eyepiece being used as a simple magnifier.

A telescope is used for viewing distant objects. The front, objective lens will produce a real image that is then viewed by the eyepiece as a simple magnifier. The total magnification is given by

M = f_obj / f_eye

where f_obj is the focal length of the objective lens and f_eye is the focal length of they eyepiece.