The eye and optical instruments

The eye: a natural optical system

The human eye works like a camera. The lens acts as a converging lens, forming a real, inverted image on the retina (the screen). The brain then ‘right-side-up’ this image mentally.

The lens: a variable focal length

Unlike a laboratory lens, the focal length of the lens can change through accommodation: the ciliary muscles alter its curvature to focus on objects that are near or far.

Visual defect	Cause	Correction
Myopia	Image forms in front of the retina	Diverging lens
Hyperopia	Image forms behind the retina	Converging lens

The camera: a simplified model

A camera replicates the principle of the eye with a lens (converging lens) that forms a real, inverted image on a sensor. Unlike the eye, focusing is achieved by moving the lens, not by changing its focal length.

The magnifying glass: an enlarged virtual image

When an object is placed between the object focus F and the optical centre O of a converging lens (distance < f'), the image produced is virtual, upright and magnified: this is the principle behind the magnifying glass, which is widely used in biology to observe small details.

Practical example

A magnifying glass with a focal length f' = 4 cm, held 2 cm from a postage stamp, produces a magnified virtual image which the eye can comfortably observe by looking through the lens.

Common pitfall

Do not confuse the image formed on the retina (real, inverted) with our conscious perception of it (righted by the brain): the eye’s optical system obeys the same laws as an ordinary converging lens.