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Old Dec 6, 2005, 12:26 AM   #1
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I have a little question here that no one (so far ) has been able to answer satisfactorily. It involves the ability of a digital camera to operate in low light conditions. From what I gather a camera with a 1/2.5" 3 MP sensor will have better low light capabilities than a 1/2.5" 6MP camera, due to the larger pixel size on the sensor; everything else being equal. Now, if I were to bump down the resolution on the6 MP camera to 3MP, would I now have the same low light capabilities as the 3 MP? I remember reading the specs of some cameras that, when shooting in low light conditions, do so (automatically) ata lower resolution. This got me wondering. Anybody have any thoughts on the matter? ...Willy.
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Old Dec 6, 2005, 7:13 AM   #2
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The photosites on the sensor don't get any bigger, so I don't think you could expect better low-light performance. The signal still has to be amplified as much (per photosite) as if you used the full 6 MP.
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Old Dec 6, 2005, 9:21 AM   #3
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Tom LaPrise wrote:
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The photosites on the sensor don't get any bigger, so I don't think you could expect better low-light performance. The signal still has to be amplified as much (per photosite) as if you used the full 6 MP.
This doesn't appear to be true, at least not always. For example, the Kodak 850 is able to operate at ISO 800 at the reduced resolution of 1.2 Mpixels. I presume this is because they are summing 4 pixels into 1 to achieve a higher signal, with the signal to noise improving at the square root of the increase, or a factor of 2 in this case -- allowing the 5 MP at ISO 400 to render an image at 1/4 the number of pixels at ISO 800. I think the OP was asking if this was the approach that was always taken with down-resed images on a camera. I believe that the answer is, "No," but I'm not positive.


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Old Dec 6, 2005, 9:24 AM   #4
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sneakyweasel wrote:
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...everything else being equal. ...
It never is, but if it were: yes, you can use a bit higher ISO since "averaging" pixels reduces noise a bit.
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Old Dec 6, 2005, 10:27 AM   #5
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[a "photosite" is the part of the sensor which is sensitive to light. 1 photosite is not equal to one pixel. But the sensor surface is covered with photosites and logic is used to merge the data from all of them to produce the image. In fact, a photosite is sensitive to a section of the color spectrum (red, green, or blue).]

I'd generally agree with the comments above.
If the camera does nothing other than use 3MP-worth of pixels, then it wouldn't be better.

If they do some tricks like merging the results of different pixels in some funky way, that might work. It wouldn't be as good, as you'd still have all the inherent problems that come from smaller photosites.

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Old Dec 8, 2005, 1:23 AM   #6
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The reasonyou can pushsome cameras to higher ISO numbers when resolution is decreased is simple - at low resolution, noise isn't as visible . Unfortunately, neither is anything else.

Theonly way to determine 'light-gathering' ability is bya camera's (or lense's)speed, measured by it's maximum aperture. Adjusting resolution has no effect.
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Old Dec 8, 2005, 11:28 AM   #7
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toshi43 wrote:
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The reasonyou can pushsome cameras to higher ISO numbers when resolution is decreased is simple - at low resolution, noise isn't as visible . ...
That is part of it, but not all. Shoot a gray card at various ISO/shutter speed combinations. Pick a fairly noisy one. The Standard Deviation of the histogram is a reasonable measure of noise: if there were no noise, the histogram would be a sharp peak - I think at RGB=128/128/128 if well exposed. You will find that the SD decreases with downsizing.
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Old Dec 8, 2005, 12:50 PM   #8
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BillDrew wrote:
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toshi43 wrote:
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The reasonyou can pushsome cameras to higher ISO numbers when resolution is decreased is simple - at low resolution, noise isn't as visible . ...
That is part of it, but not all. Shoot a gray card at various ISO/shutter speed combinations. Pick a fairly noisy one. The Standard Deviation of the histogram is a reasonable measure of noise: if there were no noise, the histogram would be a sharp peak - I think at RGB=128/128/128 if well exposed. You will find that the SD decreases with downsizing.
That's because the amount of total data in the file decreases. Noiseis data, so it will decrease proportional to the reduction in resolution.
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Old Dec 8, 2005, 8:39 PM   #9
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toshi43 wrote:
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That's because the amount of total data in the file decreases. Noiseis data, so it will decrease proportional to the reduction in resolution.
That would be true if the downsizing was done by simply throwing away some of the pixels, and some downsizing methods might do that. However, if the pixels are "averaged" the random noise should be decreased by about the square root of the ratio of pixel reduction, i.e., downsizing from 6Mp to 3Mp should decrease the noise by a factor of about 1.4 (square root of two). So I'd expect that the ISO could be increased by a factor ofabout 1.4 to have the same noise level. Not a big gain in ISO vs the trade-off against resolution, but real.

I have done some experiments, and given that a gray card does not produce a delta fucntion histogram, the results are pretty much as outlined.
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