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Old May 25, 2008, 11:35 AM   #1
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Based on what I have read in this FORUM so far, I know there are quite a fewknowledgeable peoplehere and I would like to raise a question that applies to all digital cameras including DSLR and P&S.

When we set a high pixel camerato a lower pixel frame(for example setting a 3072x2304 camera to shoot 1600x1200), we are actually using every 4 pixles as one. Why wouldn'tthese pixels share (add up) their light for a four times brighter picture? I have taken low-light pictures at different pixel settings with my Canon A710IS and do not see any difference in exposure. This would be a great feature in low-light or sports scenes. Is it just a matter of time to see this feature or is there a fundamental technical barrier?

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Old May 25, 2008, 12:26 PM   #2
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Actually, when you reduce the resolution setting of your camera, it doesn't add the values of the pixels together, it averages them. So, no, it won't increase the low-light performance.

And because of the Bayer Filter that is used with image sensors, what you describe would be quite difficult to implement. But Kodak and others are working on ways to improve the light sensitivity of image sensors, so the situation may change shortly.
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Old Jun 20, 2008, 8:55 PM   #3
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TCav

Thanks for your insight. After trying anything I could to improve the low-light performance of my Canon A710IS camera, I finally gave up and bought a Fuji f40fd. It is so much better than Canon for low light. I wish I had made this move much earlier.

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Old Jun 21, 2008, 9:20 AM   #4
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TCav wrote:
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Actually, when you reduce the resolution setting of your camera, it doesn't add the values of the pixels together, it averages them. So, no, it won't increase the low-light performance.
...
TCav and I have had this disagreement before: I will say that averaging the pixels will improve your low light perfomance. Since one major problem with low light performance is noise at high ISO - what high ISO means depends on your camera. If the camera does a simple ageraging to reduce the resolution, that should reduce the noise by a factor of 1.92 (square root of the pixel ratio). A reduction that sounds better than it is.

You can test that idea yourself, measuring the noise by the width of the histogram when shooting a flat, uniformly lit, monochrome subject like an outside wall on a heavily overcast day with high ISO (high shutter speed). The reduction will be a bit less than 1.92 because no real object is perfectly monochrome nor uniformly lit. You could use the histogram of the same object shot at low ISO (slow shutter speed) so the wall's imperfections dominate the noise.

Where TCav and I likely agree is that whatever improvement you get by the camera downsizing the image, you are almost certian to get a better improvement by shooting at the highest resolution then using noise reduction software before downsizing.

Getting a better camera will produce better results than any of the above. But whatever camera you have, you will always strain at the limits in some situation. A situation you wouldn't have even though about trying a shot with your old camera.
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Old Jun 21, 2008, 11:52 AM   #5
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But the OP presumes that when you reduce the resolution by 1/4, the pixels are added together which should produce better low light performance, and that's not true. The pixels are not added but averaged so there is no inherent improvement in low light performance like the OP is expecting to see.
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Old Jun 21, 2008, 7:17 PM   #6
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Anour wrote:
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...
Why wouldn'tthese pixels share (add up) their light for a four times brighter picture?
...
Stretching the histogram via levels will always alow any photo to be made 4 times brighter - though there are very few that would stand up to that kind of treatment. Not at all sure I would want any kind of function that made things four times brighter - I have enough blown highlights as it is.

So TCav is right - no camera does that. I would add that you probably wouldn't want any camera that did.


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Old Jun 21, 2008, 9:04 PM   #7
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Thanks folks. I think I have to live with the noise challenge in P&S, at least for the next few years.

I have been comparing f40fd with Canon A710 IS in a variety of conditions. My conclusion, so far, is that f40fd is much better when there is not enough light (for Canon) but when there is enough light, Canon's sharpness is much more impressive.

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Old Aug 28, 2008, 7:24 PM   #8
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TCav & BillDrew,

I have started to see references to"binned pixels" available on high-end canon and even the upcoming G10. The description says it processes some neighbouring pixels to reduce noise. Is this, in effect, same asadding their lights or output volts to increase SNR? Would it help to take pictures in lower resolution settings with these cameras?

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Old Aug 28, 2008, 9:10 PM   #9
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Anour, I think we can answer you with a very firm and emphatic maybe. It is certainly possible to trade off resolution against noise, but it does take a fairly large drop in resolution to give a good decrease in noise. I am pretty sure noise will go down as the square root of the resolution (pixel count) decrease so you will have to drop the pixel count by a factor of four to drop the noise by a factor of two. How that is implemented is yet another question.
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Old Aug 28, 2008, 9:19 PM   #10
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No. This is the averaging in hardware. It increases the signal to noise ratio, but it doesn't increase the signal.

First, the total number of photons that strike the image sensor doesn't change. The only way to alter that is with the aperture and shutter speed.

Second, between one half and three quarters of the photons that strike the image sensor are blocked by the Bayer Filter.

"Binning" only affects the SNR, not the signal.

The only way to increase the signal is to open the aperture, lengthen the exposure, or change the Bayer Filter so it lets more photons through.
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