المغلاق أو الغالق - ١ - طريقة عمل آلة التصوير .. كتاب التصوير الضوئي

Shutter or shutter - 1 - How the camera works .. Book of photography

Shutter or shutter

At the beginning of the time of photography, the time required to take a picture was very long due to the poor sensitivity of the plates and negative papers used. When the weather was bad, taking a picture took several minutes and sometimes 20 seconds in the middle of the sun. The picture was taken only by raising and placing the protective stopper of the lens. With the discovery of faster sensitive surfaces, the time to take the picture became shorter. At the end of the nineteenth century, the photographer had to use a shutter that gave correct durations for taking pictures between 1/second and 1/25 second.

The first automatic shutter was the hinged shutter: a simple metal plate perforated with a circular hole that passes in front of the lens. To obtain the different durations of taking the picture, the diameter of the holes had to be adjusted. Several models of shutter were proposed, but none of them met the requirements of photography in the twentieth century. In the 1920s, it was possible to easily achieve speeds of 1/200 or 250 sec, and in 1930, 1/500 or 1/1000 sec. There were only two types that could take pictures for short periods: the pyrophoric shutter and the central plate shutter. Today, most pyrophoric shutters can give 1/1000 or 1/2000 sec. There are actually problems with speeds above 1/500 sec, which are problems of mechanical spring tension and shutter inertia. The actual speed is often lower than the speed indicated on the speed cylinder.

Pyrophoric shutter

This type of shutter, which is equipped with most reflex machines and 24 × 36 machines with interchangeable lenses, is located as close as possible to the pyrophoric surface (hence its name). It consists of a system of two curtains and axes of attraction. The first curtain has a rectangular opening of the same size as the film. The second curtain, which is driven by strips, covers part of this opening to form a slit with an adjustable width. The narrower the slit, the shorter the image capture. For instant machines between 1/30 (or 1/60) and 1/1000 seconds, the slit capture that matches the length of the film surface gives the first curtain exposing the entire film area at once before the second curtain begins its movement.

- Before the image is taken, the film is protected by the first curtain.

- When the first curtain begins its movement, a part of the film is exposed to light.

- The first curtain continues its progress, then the second curtain begins its movement: The width of the slit determines the duration of exposure to light.

- The width of the slit remains constant during exposure to light.

- After the slit passes in front of the film surface, the film has been exposed to light for a period equal to the indicated speed.