Applications of lenses: the eye, the camera, the magnifying glass

The eye, a natural optical system

The eye can be modeled as a converging lens (the crystalline lens, together with the cornea) that forms an image on a fixed screen: the retina. Unlike a camera, the eye cannot move its "sensor": it is the focal length of the crystalline lens that varies through accommodation (contraction of the ciliary muscles), so that the image always stays in focus on the retina, whether the object is near or far.

The camera

A camera uses a converging lens (the lens/objective) that forms a real, inverted image on a sensor. Here, it is the distance between the lens and the sensor (the focusing) that varies, not the focal length of the lens itself.

The magnifying glass

The magnifying glass is a simple converging lens used when the object is placed between the optical center and the object focal point (OA < f'). In this configuration, the image formed is virtual, upright, and enlarged: the eye, placed just after the lens, sees it as if the object were bigger.

Summary table

Instrument	Lens type	Typical object position	Nature of the image
Eye	converging (variable)	variable, focal length adjusted	real, on the retina
Camera	converging	generally beyond 2f'	real, inverted, on the sensor
Magnifying glass	converging	between O and f'	virtual, upright, enlarged

Classic pitfall

Do not say that the eye "moves its retina" to focus: it is the focal length of the crystalline lens that changes (accommodation), not the position of the retina, unlike the camera, where it is the position of the sensor/film relative to the lens that is adjusted.