Scheimpflug file
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Only a small amount of light may enter the camera and reach the film. The amount is regulated with the shutter and the stops. The shutter regulates how long the light may enter the camera. The stop or diaphragm regulates how much light enters the camera in a given time. You can compare this with a water tap. If you open it full it only takes a short time to fill a glass. If you open it only a bit, you have to let the water come out a longer time to fill the glass.
The videos here show the closing down of stops in several cameras.


The video show the closing of the iris diaphragm on a 1898 No.5 Cartridge Kodak. By moving a lever below the lens the blades of the iris diaphragm close the opening through which the light enters. (To make the diaphragm blades better visible I have removed the front element of the lens. Also I have removed the back panel of the camera to let the light in, so that you can see the closing of the blades more clearly. In a normal situation no light comes from inside of the camera.)
The second part of the video shows the stopping down from the inside of the camera. 



A more simple version of stops can be seen on the first camera in this video. It is a rotating stop on a No. 4 Folding Kodet from about 1895. It has four holes of different size in a metal disc. By rotating the disk you can choose a smaller or larger stop.

The second camera is a No. 4 Bullet from 1896 with three stops on a metal sector. By pulling a lever on the outside of the box you can change the stop. I have stopped the shutter in the open position, so you have a better view on the diaphragm that is in front of the lens.