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.


2 Answers 2


There is no global reset on all pixels simultaneously. This is the Holy Grail of sensor design that hopefully may come soon. Canon, in order to save cost, does not include a mechanical first curtain in your RP. Electronic first curtain is just a row by row read that ignores the result by dumping the charges. That row will start accumulating charges again. That is how exposure starts. The mechanical second covers the sensor so that the pixels can be read out in leisure. Because the first curtain ignores the readout, it can "move" at the same speed as the second mechanical curtain, at ~1/200 sec from bottom to top of the picture. Full electronic shutter, the second curtain must read and convert the pixels to digital. It is a lot slower than mechanical curtains. No camera makers specify how slow their electronic shutters are. They are probably a lot slower than 1/30 sec with a much narrower "slit".


I think if you read the manual you will find that the electronic first curtain/mechanical second curtain function is only available at SS's below shutter curtain transit time/flash sync (~ ≤ 1/250). And at higher SS's it is functioning as a completely electronic shutter... which is rolling readout, not global.

The rolling readout electronic shutter works by activating/deactivating/reading lines in sequence, simulating the slit opening of a fully mechanical shutter. It is basically recording one frame of video/live view output.

  • \$\begingroup\$ How does that apply to other cameras (eg Sony a6000) in which half the shutter noise is always heard even at high speeds? \$\endgroup\$ Jul 24, 2019 at 21:48
  • \$\begingroup\$ IDK what you mean by "half the shutter noise." On a mirrorless camera you can have up to three types of shutter function... silent (rolling readout), electronic first curtain, or fully mechanical (slit). The fully mechanical shutter is required to sync with flash and to eliminate the rolling shutter effect (warping). But you won't have the "other half" of the noise common with a DSLR, which is the mirror slap. AFAIK, the Sonys all have a fully mechanical shutter capability. Apparently the EOS RP does not. \$\endgroup\$ Jul 25, 2019 at 14:23
  • \$\begingroup\$ What I meant is, unless this is Canon specific, why are eg Sony cameras (unless it is a model featuring full electronic shutter) not silent at any speed? \$\endgroup\$ Jul 26, 2019 at 0:35

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