غالق المسطح البؤري .. كتاب أسس التصوير الضوئي

Second - Focal Plane Shutter

As it appears from the name, the location of this shutter in the camera is adjacent to the focal plane of the photographic lens, i.e. directly in front of the sensitive surface, and this shutter consists of two curtains of rubber cloth so that the light does not pass through, and sometimes these two are metal, and between the two curtains is an opening whose width varies, but its length is equal . For the length of the curtain (Fig. 64)

This shutter differs from the diaphragm shutter in that the latter displays the exposure of the film or sensitive plate at once, while the focal plane shutter exposes the surface of the pen or sensitive plate gradually, as it begins to expose the area opposite the aperture, and by moving the two curtains, it displays the area adjacent to it and then the next one. Thus, whenever the two curtains move, the area opposite the opening is exposed until the movement ends. The curtain is completely on the other side, so the film or the sensitive plate has all been exposed to light in successive batches

Accordingly, Valer, the number indicating the speed shown in cameras that operate with this type of creator does not indicate the time required by the opening between the two curtains in order for the distance between one side of the sensitive film to pass to the other side, but rather indicates the time during which any area is exposed. From the sensitive plate facing the hole during its movement. So if the speed we set in the camera is . From the second, this figure indicates that each area of ​​the sensitive plate is faced. The aperture has been exposed at a time is a sound of a second, but it is not indicative That the curtain moved from one side to the other in a fraction of a second, or in other words it does not indicate the time it takes to expose the entire sensitive plate, as the movement of the two curtains or the movement of the aperture from one side of the film to the other side has been exhausted.

(a) The extent of the opening between the two curtains.

(b) The speed of movement of the two curtains, or in other words, the speed of movement of the opening. If we assume that the width of the aperture is equal to 2 cm, and that the shutter has set its speed to a fraction of a second, then this means that the curtain moves at a speed of 2 cm every penny of a second, i.e. it moves for a much longer time than that. The control of the exposure time by this shutter depends on two factors: at a speed of 2000 cm per second, so if we reduce the aperture width to 1 cm while the speed remains constant, this means that the exposure speed has been reduced to half, so the exposure time becomes a blim of a second only and it was from It is also possible to control the exposure time and raise it to a speed of even one second in another way: which is to fix the width of the aperture and keep it 2 while increasing the speed of moving the two curtains to 4000 cm per second.


The time it takes to expose the entire sensitive plate


We learned that the estimated speed on the camera indicates the time it takes to expose each area of ​​the sensitive plate faced by the aperture. It is exposed to light, and now we want to know the amount of time it takes to expose the entire sensitive plate if its width is 20 cm and the width of the opening between the two curtains is 2 cm. With the following simple equation, this time can be estimated as follows: An area whose width is 2 cm needs to be exposed to light for a proper amount of time. The second... An area whose width is 20 cm needs to be exposed to light for a time of x: of a second.