Explain f stop range?

A pinhole camera is a camera without a conventional glass lens. An extremely small hole in a very thin material can focus light by confining all rays from a scene through a single point. In order to produce a reasonably clear image, the aperture has to be a small pinhole on the order of 0.5 mm (0.02 inches) or less. The shutter of a pinhole camera usually consists of a hand operated flap of some light-proof material to cover and uncover the pinhole. Pinhole cameras require much longer exposure times than conventional cameras because of the small aperture; typical exposure times can range from 5 seconds to hours or days.