It's a tool for calculate the ratio between aperture (F-Stop) and exposure time (Shutter Speed) in Computer Graphics when using a physical camera (like Vray and other's rendering engine) or in Photography.
Click on the left or the right Stop-Disc's side until matching the F-stop values and the Shutter Speed you have previously set in your camera, or physical camera if you used a rendering engine.
Up to this point, you will get all the matching values on the Stop-Disc.
Choose the new F-Stop value on the Stop-Disc.
You will get the matching Shutter Speed value needed to get the same camera esposition's starting setting.
It's very useful if you want to get the F-Stop value in order to increase the Depth of Field.