700D: Why using the mechanical shutter?

[Helmut]

Hi everyone,

I have a understanding problem how the 700D works internally.

Why is it not possible, to shoot in LiveViw continously high-res photos while the mirror is permanently locked up?

Why using the mechanical shutter at all?

If I get it right, in LV the shutter is open and the mirror is up. The light hits the sensor and the sensor data is streamed to the LCD, right?

Why not writing the complete sensor data with as many fps as possible into a buffer and then averaging for every pixel over the desired exposure time? RAM limitations?

But again, although I´m programming microcontrollers I have no knowledge yet, how a DSLR - especially the 700D internally works.

A link or a short explanation would be highly appreciated.

What is the difference to shooting a RAW video? What limits the resolution? Is it the amout of data that needs to be written? Or is it the bandwith of the internal DSP that limits the resolution? What about a RAW video with the full sensor resolution and - lets say 3 fps?

Helmut

[dmilligan]

Why using the mechanical shutter at all?

Because the mechanical shutter is better at controlling exposure than electronic shutter (mainly because it's faster). There's a finite speed that pixel values can be read out from the sensor, and rows are readout one by one. You have to leave the pixel "on" (i.e. collecting light) until you read it out, and it takes a lot of time to read out the entire sensor (measurements here). Or you can read out the entire FOV of the sensor, but skip some pixels (you can readout faster, but resolution is reduced, this is what LV does).

The mechanical shutter's purpose is basically to cover pixels until they're all finished being readout to achieve short shutter speeds and reduce rolling shutter effects.

With ML you can take full resolution photos without the mechanical shutter, but there are shutter speed limitations (fastest shutter speed is ~1/6 second): http://www.magiclantern.fm/forum/index.php?topic=12523.0 (this feature is still in development, and is not part of nightly builds yet)

Or you can take LV resolution photos without the shutter (faster shutter speeds are possible, but rolling shutter is worse, and resolution is reduced): http://www.magiclantern.fm/forum/index.php?topic=5240.0

[Helmut]

Thank you for the links and the explanation, dmilligan! That helps to understand.