“Slow shutter” or "SLS" is when your cameras electronic shutter is open for longer than 1 frame of video. Normally a video camera operates at 25 or 30 frames per second. So, the camera sensor normally captures light for 1/25th or 1/30th of a second or some smaller fraction of the length of each frame such as 1/50th or 1/60th.
The sensor is then reset, captures for the next 1/25th or 1/30th and then writes the next frame and so on, creating a video sequence. With a slow shutter the sensor is allowed to capture light for more than the length of one frame, so the shutter opening spans several frames. As an example with a 16 frame slow shutter the sensor is allowed to capture light for 16 frames before creating an image.
At 30fps, where one frame lasts 1/30th of a second a 16 frame slow shutter will be open for 16 x 1/30th = 0.53 seconds. The sensor is being allowed to capture light for 0.53 seconds before getting reset. If you do not use interval record mode or time-lapse and record conventionally at 30fps, each group of the 16 video frames the camera records while slow shutter is open will contain the same image. So with a conventional video recording the image only refreshes once every 16 frames.
If however you were to use the camera interval record function and set the interval to 1 frame, each recorded frame will have a shutter opening of just over half a second. This might be a nice way to add some motion blur to your timelapse to make it flow better.
When the shutter is set to off, the cameras shutter is open for the full duration of each frame. So shutter off at 25fps means a 1/25th shutter or 360 degree shutter. If you use S&Q motion and shoot at 1fps and set the shutter to "off" the shutter will be open for the entire duration of each frame, in this case 1 second.
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