What else is happening before the first curtain on E-M1?

Started Mar 31, 2014 | Discussions thread
Anders W Forum Pro • Posts: 21,468
Re: Shutter video

lattesweden wrote:

Does anyone here know the difference between electronic shutter and electronic first curtain?

And I mean that from a timing perspective. Electronoic shutters scan row by row as we know. How does the electronic first curtain operate from that perspective? Wouldn't it have to use the same row by row mechanism as a fully electronic shutter to open the "feed" to the memory but instead of switching the feed off electronically it uses the real shutter curtain that just shuts the light out and then the electronic feed is also cut and the file saved. And the camera can after that again open the real first curtain and go back to live view?

Or does it work differently?

No it works pretty much the way you describe. An electronic shutter resets the sensor (empties the capacitors that store the charges) row by row and then reads the sensor row by row. An electronic first curtain resets the sensor row by row and then finishes the exposure row by row by having the second mechanical shutter curtain pass over them. Read-out in this case starts only after exposure is mechanically finished.

The critical difference is the time limits involved. With current sensors, read-out takes time if it is to be done well, without adding too much noise. And since read-out must be slow, the reset with current electronic shutter must be equally slow. Otherwise, the two processes wouldn't synch and exposure would vary across the frame.

Consequently, the electronic shutters we currently have on Pany bodies take between 1/10 and 1/25 s to finish the exposure across the entire frame, which makes them unsuitable in some situations (because of so-called rolling shutter or "jello" effects due to camera or subject movement during the long total exposure time). And cutting the time down to 1/25 s comes at the expense of more noise. At full quality, it's 1/15 s at best.

However, reset can be done more quickly than read-out, apparently as quickly as 1/320 s (the max speed of the EFCS of the E-M1) or faster, which is more than ten times as fast as an electronic shutter. So with an EFCS you can have some of the benefits of an electronic shutter (no blur due to the movements of the first curtain of a mechanical shutter) without the downside of "jello" effects.

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