Originally posted by denisv Is there somewhere I can read more about this?
The 5DII records up to 24 minutes. I assume that's a arbitrary limitation since if it doesn't overheat in 24 minutes, it probably wouldn't in ten hours either. Besides, with photos, the sensor would be protected by the mirror when not in use, the only difference being that the mirror isn't precisely timed.
Don't DSLRs with video capture already have this? What about the K20D with 21fps burst mode? Or, uhm, any DSLR with live-view?
The noise increases drastically with an all electronic (on chip) shutter.*This is not really important for moving images, as the noise is statistically distributed and also the resolution is low, compared to still images. The burst mode of the K20 is very low res - and thus serves as a good example of the compromises, that need to be afforded.
Live-view: some cameras, the K20 is an example, don't expose during live-vioew, but use the shutter mechanism instead! YOu will see, when using the K20 in live-view mode, that for the exposure the mirror first goes down, the shutter releases and then the mirror goes up again, to make way for the sensor to record the live-view again.
I do not say, that using an on-chip shutter, is not possible.*It obviously is. But it affords quality compromises, which many DSLR users simply wouldn't want to make.
Using the mirror instead of an additional electronic shutter is simply a question of mechanics. The mirror has a really long travel and very high acceleration, when it starts and stops its movement - and then reverses its action. If you want to use it as a shutter, the acceleration simply gets to high. That would either mean a completely oversized mirror mechanics and also very heavy mirror slap or the mirror will break apart...
The mirror-as-a-shutter question is, by the way, not a purely DSLR-related question, the same question could arise with film SLRs.
Ben