What are the physical limits of a mechanical shutter?

I've recently found myself shooting a lot of photos on the maximum speed that my camera (a Canon 600D) allows (1/4000). This is quite extreme when thinking about mechanical movements, so I assumed that the digital shutter must be used at that speed. What is the maximum speed that is possible for the mechanical movements and where does the electronic shutter kick in if at all?

P.S. My brain also tells me that there is a possibility that with the amount of light available and the speed being so high that the entire sensor might not actually have to be fully exposed( at one time) to the light source but that a small slit between curtain 1 and 2 should be sufficient.

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Just as an example, here's a YouTube video of a Canon 5D Mk II shutter at 1/2000 of a second, shot on a 2000fps camera. – Edd Jan 15 '12 at 10:29
wow awesome youtube video thanks @Edd – fluf Jan 15 '12 at 19:35
For when an electronic shutter is used, see the answers to When does the camera use the mechanical shutter and when the electronic shutter? – Esa Paulasto May 24 '13 at 6:51

The Canon 600D uses a mechanical shutter, and it does indeed go up to 1/4000th rate. There is no "electronic" shutter in Canon DSLR's that I know of. You pretty much nailed it on the head with your 'ps'...the two shutter curtains race over the sensor with a tiny slit (see 'Focal plane shutter, high speed' figure), with the second curtain a minuscule fraction of a second behind the first curtain.

As for maximum speed, I can't say for sure, but pretty high. There are many DSLR cameras that have a 1/8000th or 1/10000th shutter rate, usually "pro grade" models. The high maximum speed of this type of shutter is one of its strengths (leaf shutters, for example, tend to be limited to about 1/500th.)

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The top speed is almost infinitesimal, theoretically, but there's be a point at which small differences in speed between the front and rear curtains would make exposures inconsistent. As far as the mechanical speed goes, I remember the professional Minoltas (the 9xi series) having an X-sync of 1/300 on 35mm and a top speed of 1/12000, and that represents a lot of acceleration. As for leaf shutters, the Schneider lenses for the Phase One/Mamiya go up to 1/1600, but they'd represent the current state of the art (they're just dribbling onto the market now). – user2719 Jan 13 '12 at 8:36
Great comment, @Stan. Before "infinitesimal" is reached, though, you will get hit by some severe diffraction effects, even if you can assure the two curtains remain parallel and at a constant distance apart throughout the exposure. – whuber Jan 13 '12 at 21:52

The 600D has a focal plane (mechanical shutter) that is electronically controlled, but it a mechanical shutter, operating as you suggested (with a slit between the curtains exposing the sensor).

What you could be thinking of when you say electronic shutter is the rolling shutter that is used to record video, but not still images.

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Well, except that CCD sensors with electronic/mechanical hybrid shutters were once commonplace in DSLRs -- the Nikon D1 series used it, as does my D70. The FP shutter never goes faster than what would be the X-sync speed for a purely mechanical shutter (that is, the sensor is completely exposed at all shutter speeds). – user2719 Jan 13 '12 at 8:26