As we all know, a DSLR camera works by moving a mirror out the sensors way when taking a picture and then going back to start, not exposing the sendor. How does this process work, on an optical motion picture camera, while filming? How can the dop/camera operator see what's beeing recorded?
There are three ways that film cameras do this:
- Using a rangefinder, i.e. a viewfinder that doesn't actually view through the lens. Common on cheaper consumer format cameras such as super-8
Using a beam splitter. The ubiquitous Bolex 16mm cameras have a prism in the light path that splits the light coming through the lens, so that some of it goes to the film gate, and the rest goes to the eyepiece.
This has the advantage of being through the lens, and continuous, but at the cost of brightness for the viewer and a stop or so of exposure for the film. From personal experience, in low light it can be really hard to focus a Bolex.
Using a rotating mirror. In this setup a reflective mirror interrupts the light path to the film, similar to a DSLR, but instead of hinging up it rotates.
When the camera is stopped the mirror is parked so that it sends the light to the viewer, and when shooting it rotates, acting as the shutter. While the mirror is sending light to the viewfinder the film is being pulled down to get the next frame in position in the gate. Then the frame is exposed, during which time the viewfinder will be black. This cycle is repeated for each frame
This has the advantage that all the light available goes to expose the film, but at the cost of some flickering in the viewfinder (I found that helpful: it lets you know that the camera is running—on silent cameras it can be almost impossible to tell otherwise).