Optically perceptible objects are those that give off light, either because they themselves produce it (these are called light sources) or because they receive light from sources and then diffuse it. An optical system gathers incoming light in its opening and directs the light to form an image—i.e., a purely optical reproduction of the object from which the light originates. The image is made of light and can often be seen by placing a screen (such as a sheet of paper) in its path. In this case, the image is called "real." The light travels beyond the real image, spreading and blurring the shape.

Alternatively, the image formation may be merely presumed: this time, the screen will capture the light but not the actual shape of the object. Such images are called virtual. To be displayed—for example, on photographic film, a television-camera sensor, or the retina of the eye—an image must be real. To form a real image, the entire optical system needs to focus.

The eye is equivalent to a converging lens with a focal length of 2 centimeters. The telescope is an afocal system. When the eye is combined with the telescope, the resulting optical system focuses, forming a real image on the retina. The ratio of the size of an image generated with the telescope to the size of an image produced without an instrument is called the magnification.