[_Microft]

That's exactly the mirror that gives (D)SLR's the R ("reflex") in their name. It's in the path between sensor and lens and redirects the image towards the view finder. When taking a picture this mirror moves out of the way and the image is projected on the film/sensor instead. The advantage of that system is/was that the photographer can preview the image as it will get captured.

[throwaway22032]

Ah, so there's no shutter at all then? As in, the mirror itself forms the shutter?

For some reason I figured there were both. Makes sense I guess.

[RobotToaster]

No, in most* cameras there's still a shutter behind the mirror, the mirror can't move that fast so would limit the top shutter speed too much.

*The Ihagee Exa is the only one I know of that used a mirror guillotine shutter.

[actionfromafar]

I think (some? most?) motion picture cameras used a rotating mirror for shutter.

[smogcutter]

Correct, it’s like a mirrored fan blade and rotates continuously.

Ironically, with mirror reflex system what you see through the finder are precisely the moments not captured on film. Very quick events, muzzle flashes for example, can be missed entirely.

Notably, some Bolex cameras (and others?) used a beam splitter system where you do see what the film sees, with no flicker from the shutter. The tradeoff is a dimmer image in the finder, and you’d need to overexpose to compensate for the lost light.

[actionfromafar]

Curiously, there was also a Canon SLR with a beam splitter.

https://en.wikipedia.org/wiki/Canon_Pellix

[numpad0]

Technically, many dSLRs use beam splitter principle and secondary mirrors to feed the image to AE/AF device at the bottom side of mirror box.

[RobotToaster]

Sony also made some SLR that used beamsplitters https://en.wikipedia.org/wiki/Sony_SLT_camera

[numpad0]

That will expose the bottom side of the film longer than the top, unless there's a second mirror that folds up :)

ref: https://youtu.be/SynB6Qypk4c?si=wmLgwjTQNeDtqCYl&t=64

[ginko]

There is a separate shutter.