It is an optical instrument with which is used to form a real image of an object on a screen.It is consist of a lens or lens system placed in the front of a dark box of folded leather. The screen is a ground glass plate which is placed on the opposite side of the lens.

The distance between lens and screen can we change to get a sharp and well defined image of the object on the screen. This screen is a sensitive plate which is exposed to light on the for a short time. The light is controlled by a shutter which is screen having a circular aperture at its center.

The diameter of this aperture can be varied to control the light. The time of exposure of light is usually small fraction of a second. The exposed plate is care fully taken to a dark room where it is washed in a solution called the developer.

Then it is washed in another solution called the fixer. In this way we get what is called the negative, as bright as part of the objects appear dark in the image form on the plate. Another sensitive plate is placed below the negative in a  dark room and is exposed the light from a suitable time.The new explode plate also treated with the developer and fixer.

In this way we get different what is dark in the negative becomes bright and bright becomes dark in the positive or print.In this way the original colour of the object or landscape are explored on the print or the positive.