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10/09/2009 02:32:16 AM · #1 |
I've tried doing a couple of searches on Google about this, but it's difficult to find an answer because I keep getting the "Frames Per Second" rate, but I've often wondered if digital camera sensors have a sample rate of x frames per second while the shutter is open.
The reason I have thought this is that often when I do a panning shot with some strong lights in the background, the light is broken up into dots rather than a continuous streak. I am aware that this could be because the lights naturally flicker due to the electrical current, but I'm sure I've also observed this in other lights that shouldn't flicker such as car lights (though I could be wrong).
I saw an example of this in the recent free study with  briantammy's image (see below) so my question is, does the camera continually sample while the shutter is open, or is it more like film where it absorbs light as it arrives?
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10/09/2009 03:17:18 AM · #2 |
Normal image sensors collect light during the full shutter time, so the effects you've seen are most likely due to flickering lights.
In a CCD, the pixels are connected to their neighbours, and data is read out at the end of the row by shifting the electrons in all pixel by one step, reading one pixel at a time as it "falls out" of the row pixel by pixel. Since the pixels are still sensitive to light, the shutter must be shut during this shifting process to avoid smearing (Unless the CCD is of "interline" type - and still image cameras usually are not).
CMOS sensors can address individual pixels and do not have to worry about smearing. But, just as for the CCD, it takes quite some time to read out all pixels of a high resolution sensor. In theory, a CMOS camera could however do multiple samplings within one shutter opening if the shutter time is very long. I don't know if there are any that actually do this to e.g. suppress noise in extremely long exposure images.
Here's some info on CCD sensors: //en.wikipedia.org/wiki/Charge-coupled_device
CMOS sensors: //en.wikipedia.org/wiki/Active_pixel_sensor |
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10/09/2009 05:45:00 AM · #3 |
| OK, good explanation, thanks for that, I guess it is just the flicker of lights after all. |
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