How does electronic first-curtain shutter work with a mechanical second curtain?
Asked 7/23/2019
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On cameras like the Canon EOS RP, I understand that at fast shutter speeds a mechanical focal-plane shutter exposes the sensor with a moving slit rather than fully opening and then closing instantly. With electronic first-curtain shutter (EFCS), the first curtain is electronic but the second curtain is mechanical.
Does EFCS start exposure by resetting the sensor row by row so that the electronic "first curtain" effectively travels across the sensor at the same rate as the mechanical second curtain? Or is it done some other way? I'm also trying to understand why the second curtain still needs to be mechanical, and how this differs from a fully electronic shutter.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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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".
Originally by user98005. Source · Licensed CC BY-SA 4.0
user98005
5y ago
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Yes—EFCS is effectively a rolling electronic start to exposure, not a global simultaneous start across the whole sensor.
In this mode, the camera begins exposure line-by-line by resetting/clearing rows in sequence. Those rows then start collecting light again. Because that "start" operation can be done quickly, it can be timed to mimic the travel of a mechanical first curtain.
The mechanical second curtain then physically covers the sensor, after which the camera can read the pixels out more leisurely. That mechanical stop is useful because the sensor generally cannot instantly stop and hold all pixels at once for a true global exposure end.
So your revised understanding is basically correct: EFCS behaves like an electronically simulated first curtain that moves across the sensor in step with the shutter travel, while the second curtain is mechanical.
A fully electronic shutter is different: both start and end are done line-by-line via sensor readout, which is slower and causes rolling-shutter behavior. Some cameras may only use EFCS up to around the shutter’s curtain-travel / flash-sync range, and above that may switch to fully electronic rolling operation.
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