A lens is a device for either concentrating or diverging light, usually formed from a piece of shaped glass. Analogous devices used with other types of electromagnetic radiation are also called lenses: for instance, a microwave lens can be made from paraffin wax.

There are differently shaped lenses. A concave lens makes objects appear smaller, while a convex lens makes them appear bigger.

Your eye projects an upside-down image of what you are looking at on to the retina. Special cells in the retina transmit information about the light that hits them to the brain, which then builds up a picture we can see. The lens in the eye ensures that we see a sharp and well-focussed image.

Short sight.  The eyes, in people who suffer from short sight, focus light from a distant object in front of the retina. A concave lens will correct this.

Long sight.  The eyes, in people who suffer from long sight, focus light from a near object behind the retina. A convex lens will correct this.

A camera is a device used to take pictures (usually photographs), either singly or in sequence, with or without sound recording, such as with video cameras. A camera that takes pictures singly is sometimes called a photo camera to distinguish it from a video camera. The name is derived from camera obscura, Latin for "dark chamber", an early mechanism for projecting images in which an entire room functioned much as the internal workings of a modern photographic camera, except there was no way at this time to record the image short of manually tracing it. Cameras may work with the visual spectrum or other portions of the electromagnetic spectrum.

A lens is used in a camera to allow both a large hole (aperture) and a sharp image. The lens bends the light rays from a single point on the object back to a single focussed point on the film.

Single-lens reflex camera.  A modern camera directs the light from the lens to the viewfinder, via a mirror. When the button is pressed, the mirror flips up, the shutter opens and the focussed light falls on the film.

Pinhole camera.  A pinhole camera can take a very sharp photograph. This is because the pinhole is very small and only lets through a tiny amount of sharply-focussed light. It can take several minutes before enough light has reached the film for a picture to form.

Pinhole image - focussed.  Like the eye, a pinhole camera projects an upside-down image. The pinhole is very small and only lets through a tiny amount of sharply-focussed light.

Pinhole image - unfocussed.  When the pinhole is large, the light from a single point on the object gives a blurred image on the film.

This common microscope can be found in many school laboratories. A mirror is used to direct light through the specimen.

A microscope (Greek: micron = small and scopos = aim) is an instrument for viewing objects that are too small to be seen by the naked or unaided eye. The science of investigating small objects using such an instrument is called microscopy, and the term microscopic means minute or very small, not easily visible with the unaided eye. In other words, requiring a microscope to examine.

The most common type of microscope—and the first to be invented—is the optical microscope. This is an optical instrument containing one or more lenses that produce an enlarged image of an object placed in the focal plane of the lens(es).