the double shutter speed rule ruins your video

Started 5 months ago | Discussions thread
Sean Nelson
Sean Nelson Forum Pro • Posts: 15,001
Re: the double shutter speed rule ruins your video

John Brawley wrote:

Here is a break down of that over a single second...

1/48th - Shutter is open

1/48th - Shutter is closed

And then within each OPEN shutter cycle....

Shutter is open for 1/48th of a second...

1st line is recorded

I believe it's a little more complicated than that, because in fact there is no physical "shutter" that closes in video mode - the entire sensor is being exposed to light continuously. The key is in how the charges that have built up in the photosites are cleared, new photons are allowed to accumulate, and then the accumulated values are read out from each row of the sensor.

As an example, let's say it takes 1/30th of a second to read out all of the rows in the sensor, a time that's consistent with a sensor that has a 33ms readout time. Let's further assume that you're using a 1/60th second exposure, which is consistent with a 180 degree shutter at 30fps. (the 30fps frame rate is mostly irrelevant to this discussion). And for sake of simplicity let's assume that the sensor has a resolution of 1000 vertical pixels, or rows.

What happens at the start of capturing a new video frame is that the topmost "row" of the sensor is reset - the accumulated charges in all of its photosites are drained so that from this instant forward a new exposure is started for all of the "pixels" in that row. From this point on charges accumulate in the photosites for the row as photos hit the sensor.

Then the next row is cleared, then the next row, and so on. By the time the 1/60th exposure time has been reached, the first 500 rows in the top half on the sensor have been cleared.

Now the sensor starts to read out the topmost row - each value represents the number of photons that have hit a photosite in the row since it was cleared out 1/60th of a second ago.

At the same time, the 501st row is cleared, thus starting its exposure.

Next, the 2nd row of the sensor is read as the 502nd row is cleared.

Then the 3rd row is read as the 503rd row is cleared.

And so on all the way down to the bottom of the sensor.

This entire process is repeated 30 times a second.

In this hypothetical sensor, it takes a whole 1/30th of a second to read out the entire sensor, so its maximum frame rate is 30fps. This means the sensor actually has to start exposing the top part of the second frame while the bottom part of the first frame is still being read out. At a 1/60th exposure, the topmost line is reset to start the 2nd frame's exposure when the 501st row of the first frame is being read. This is why the light from a strobe can appear in two consecutive frames of the video - the top portion of one frame and the bottom portion of the other.

Logically speaking this is equivalent to a slow physical focal plane shutter whose curtains take 1/30th of a second to travel from the top to the bottom of the frame - but it's all done electronically instead..

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