Rolling shutter on Electronic vs First Curtain
Rolling shutter on Electronic vs First Curtain
I'm trying to understand why rolling shutter is a problem with electronic shutters, but not with electronic first curtain shutter modes. I've done lots of digging trying to get to the underlying reason for the difference, but I haven't had any luck, just various people talking about the fact it is a problem.
So fediverse photogs, who can help me with an answer?
They're entirely different mechanisms. Electronic front curtains work entirely differently than a fully electronic shutter. I'm not sure how much you understand about shutters and their mechanics, or the way camera sensors work, so I'll try to take a middle ground explanation here....
An electronic shutter blanks every "pixel" in the sensor, and the iterates over the pixels and reads their values. The blanking signal can easily be synced, and is effectively instantaneous. Iterating over the pixels takes time - 48MP at 10bits is a lot of fucking data, and it is extremely complicated for the sensor to be able to read out that many pixels, the memory buses to transfer the data, the processing units to process the data, and the storage mechanism to write that data. It's not a long time, but it takes fractions of a second, so it approaches shutter speeds. This means the whole mechanism runs "line by line" so the later lines are read "later in time" from the time you clicked the shutter button than the earlier lines. Even if you sync the blanking signal to match this later read time, in order to keep the "exposure time" constant across pixels, the pixels are still read in order and the later read pixels are capturing a later point in time than the earlier read pixels. We just can't read from an entire sensor within 1/1000th of a second.
That being said, electronic front curtains are way simpler. You blank everything together, then you snap the rear curtain closed, then you read the pixels. Even if the later pixels are read later, they are behind the rear curtain, so they aren't exposed to any of the "later light" - the exposure data in all the pixels are "saved" from the same point in time. Even though they're read later, the rear curtain ensures they never received light beyond a specific point in time. As long as you read the data after that point, there's no noticeable variance in the point in time that the pixels were exposed to the picture.
Again, this is a bit of a simplification - there is a lot that goes on in the syncing process between curtains to be able to obtain really fast shutter speeds, but the concept is essentially as above - a fully electronic curtain is constrained by the read time of the sensor, but even an electronic front and mechanical rear shutter can use the light-blocking properties of the physical shutter to ensure better synchronicity between the different sensor lines.
14Thanks. That's what I was looking for. I understood the mechanics of the different shutter types, but I couldn't understand why it made a difference. The key part I was missing was the fact that the sensor remains open and collecting light after the rear curtain has been raised, which mitigates the skew from the line by line processing of the sensor.
2Of course! Glad I could help!
Yeah, the sensor could still be in the reading phase and "collecting light" while the shutter is closed, but since the shutter's closed, there's no light getting to it so makes no difference, and therefore any "read delay" doesn't cause issues.
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So the electronic rear curtain shutter is better for rolling shutter problems, but the trade-off is that you still need the mechanical shutter (which causes wear and tear)?
2I wouldn't worry too much about shutter wear - they're rated really highly, so while it is wear, I don't know that you'd realistically ever encounter an issue unless you were using the same body for tons and tons of work, and not replacing it for a very long time.
In my experience, the tradeoff is more "rolling shutter problems" vs "the loud noise of the shutter click". There are times and places where the shutter click can ruin moments, be outright forbidden, or will distract people. There are others where fast motion is occurring and the rolling shutter is an issue.
To me it's more of a question of "do I need to be silent? Then I need to use electronic". But if not, I may as well use electronic front/physical rear or just both physical. I think the "electronic front or mechanical front" is the more tricky question. I just don't use full electronic unless really necessary because I'm more worried about ruining shots than I am about my camera lasting 20+ years.
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