I previously incorrectly understood how the shutter curtains work. I thought they move at so high speed that e.g. at 1/4000 s shutter speed, the first curtain first opens very quickly, the shutter is open for 1/4000 s and the second curtain closes very quickly.
However, I was recently told that at 1/4000 s shutter speed, the curtains move really slow and there is only a small moving slit between the first and second curtains.
My camera, a Canon EOS RP, however, has electronic first curtain shutter. The second curtain is mechanical.
How does this electronic first curtain shutter actually work?
My first understanding of it was simply that it globally connects all of the CMOS photosites to ground (so all pixels have the RST line active using the terminology of Wikipedia's CMOS sensor page), and then the RST is deactivated simultaneously across the whole sensor area.
However, if the second curtain is moving slowly, obviously the first curtain has to be moving slowly too.
So, my current understanding is that the RST lines are first active, and then gradually deactivated on a row-by-row basis, so that the row where reset is to be deactivated moves exactly at the same rate as the mechanical second curtain moves.
Is this current understanding correct? I think this current understanding would avoid the severe rolling shutter effect of electronic second curtain. The only problem would be to match the movement of the electronic first curtain very accurately to the movement of the mechanical second curtain.
This current understanding would also explain why the second curtain is mechanical, i.e. CMOS photosites have no way to be stopped while maintaining the current charge, apart from covering them with a mechanical curtain, and you can't read the whole sensor area in few milliseconds.