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What's the difference between long exposure where the shutter(on real cameras) is open for a certain period of time, and recording of a video on a webcam for that same time?

Isn't the sensor exposed to the same amount of light in both cases?

And if so, could it be theoretically possible to produce/generate a long exposure night photo shown below by stacking frames from that recorded video?

enter image description here

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A sequence of video frames would not be able to replicate the image you posted. You MUST have a long exposure, long enough such that cars effectively "disappear", leaving behind only their light trails. Stacking fast still frames would not be able to eliminate the cars. –  jrista Jun 22 '13 at 21:44
    
@jrista Okay, I'll bite. Why wouldn't that work? –  mattdm Jun 23 '13 at 12:42
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1 Answer

up vote 3 down vote accepted

The biggest difference is that a single, long exposure, photo is a single read off the sensor and the video stacking isn't, it's a sequence of frames, probably taken 25 or 30 times in a second.

In terms of single versus stack, well, I can think of a few things to bear in mind with video:

  1. Video frames are usually smaller in size (length/width) so level of detail for same field of view will be sadly lacking.
  2. You may need hundreds, or more, video frames to get the same exposure time.
  3. You may need to boost the ISO to get a reasonable amount of data in the short sequence of time between reads on the sensor.

All in all, I'd suggest you'll get much better results with a single shot. Stacking, by the way, is common in astrophotography, though, as @jrista notes in the comments, you can't get the long exposure effects of light blur with the source removed as a result of speed of movement.

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Some people do use stacked high def. video frames for astrophotography. It is a "newer" technique from a number of years ago, but when you have the ability to stack thousands of frames, especially when they are 1080p, you can actually get some pretty good results. Stacking, however, would be unable to replicate the sample shot, which shows car light trails, but eliminates the cars themselves due to the nature of long exposure. –  jrista Jun 22 '13 at 21:45
    
Good, point, I'll update... –  John Cavan Jun 22 '13 at 21:54
    
Thanks guys, great points! All this made me think a lot about digital photography. I took many things for granted... But now I see I don't even understand exactly how electronic imaging sensors work, let alone how does one get long exposures with them. It's easy to understand for analog film. It's much simpler process there. –  Ivan Kovacevic Jun 22 '13 at 22:11
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