Although a range of different shutter devices have been used during the development of the camera only two types have been widely used and remain in use today. The Leaf shutter or more precisely the in-lens shutter is a shutter contained within the lens structure, often close to the diaphragm consisting of a number of metal leaves which are maintained under spring tension and which are opened and then closed when the shutter is released. The exposure time is determined by the interval between opening and closing. In this shutter design, the whole film frame is exposed at one time. This makes flash synchronisation much simpler as the flash only needs to fire once the shutter is fully open. Disadvantages of such shutters are their inability to reliably produce very fast shutter speeds ( faster than 1/500th second or so) and the additional cost and weight of having to include a shutter mechanism for every lens. The focal-plane shutter operates as close to the film plane as possible and consists of cloth curtains that are pulled across the film plane with a carefully determined gap between the two curtains (typically running horizontally) or consisting of a series of metal plates (typically moving vertically) just in front of the film plane. The focal-plane shutter is primarily associated with the single lens reflex type of cameras, since covering the film rather than blocking light passing through the lens allows the photographer to view through the lens at all times except during the exposure itself. Covering the film also facilitates removing the lens from a loaded camera (many SLRs have interchangeable lenses). In photography, a shutter is a device that allows light to pass for a determined period of time, for the purpose of exposing photographic film or a light-sensitive electronic sensor o light to capture a permanent image of a scene. A shutter can also be used to allow pulses of light to pass outwards, as in a movie projector or signal lamp.Camera shutters can be fitted in several positions: Leaf shutters are usually fitted within a lens assembly (central shutter), or more rarely immediately behind (behind-the-lens shutter) or, even more rarely, in front of a lens, and shut off the beam of light where it is narrow. Focal-plane shutters are mounted near the focal plane and move to uncover the film or sensor. Behind-the-lens shutters were used in some cameras with limited lens interchangeability. Shutters in front of the lens, sometimes simply a lens cap that is removed and replaced for the long exposures required, were used in the early days of photography. Other mechanisms than the dilating aperture and the sliding curtains have been used; anything which exposes the film to light for a specified time will suffice. The time for which a shutter remains open (exposure time, often called "shutter speed") is determined by a timing mechanism. These were originally pneumatic (Compound shutter) or clockwork, but since the late twentieth century are mostly electronic. Mechanical shutters typically had a Time setting, where the shutter opened when the button was pressed and remained open until it was pressed again, Bulb where the shutter remained open as long as the button was pressed (originally actuated by squeezing an actual rubber bulb), and Instantaneous exposure, with settings ranging from 1" to 1/500" for the best leaf shutters, faster for focal-plane shutters, and more restricted for basic types. The reciprocal of exposure time in seconds is often used for engraving shutter settings. For example, a marking of "250" denotes 1/250". This does not cause confusion in practice.