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Old Dec 20, 2007, 1:42 PM   #1
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Everything I have read on the web seems to attribute noise in compact cameras to the small amount of surface area per pixel on the sensor. More specifically, articles usually say that more photons will hit a larger pixel.

I'm confused by this. Say you have sensor A and sensor B, both 10 megapixels but B is much larger than A. Lens C set at aperture D should let photons through at the same rate all the time. So if you use this lens assebly to focus light across the full surface of each sensor won't you have the same number of photons striking each pixel on both sensors A and B? In this case, the same number of photons will hit each pixel regardless of pixel size.

So is the problem really the sensor size? Wouldn't a larger lens assebly with more surface area on the lens and a larger physical aperture size allow plenty of light to hit a small sensor? Is it possible that large enough lens assemblies are just too impractical on compact cameras and that a large enough physical aperture would ruin your depth of field?

Sorry if this has been discussed already, but I haven't found an answer anywhere. Can anyone comment?
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Old Dec 20, 2007, 2:33 PM   #2
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Noise isn't a factor of the sensor size, it's a factor of the pixel density. The tighter you pack the pixels in, the more likely one of them is going to over-react as a result of stimuli from adjacent pixels.

You're correct that increasing the size of the sensor reduces noise. That's why dSLRs, which have among the largest image sensors, have the least noise, all other things being equal.
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Old Dec 20, 2007, 3:44 PM   #3
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The larger the photosite the better the signal to noise ratio and the less noise in the photo. Bigger sensors have room to have bigger photosites and still get a decent amount of pixels.
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Old Dec 20, 2007, 5:36 PM   #4
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Yes, poor signal results in more noise, just unplug the aerial from

the TV set and watch what happens,larger collectors in the sensor

help keep the signal at a higher voltage and on top of background

noise, as well as being spaced out better in larger sensorsto help

avoidstimulation from other collectors in the sensor...... Other types

of noise reduction usually result in loss of image detail............musket.
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Old Dec 20, 2007, 9:08 PM   #5
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mogumbo wrote:
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I'm confused by this. Say you have sensor A and sensor B, both 10 megapixels but B is much larger than A. Lens C set at aperture D should let photons through at the same rate all the time. So if you use this lens assebly to focus light across the full surface of each sensor won't you have the same number of photons striking each pixel on both sensors A and B? In this case, the same number of photons will hit each pixel regardless of pixel size.
If the light level is the same, the number of photons strikingthe larger area will be greater.

One of the easiest ways to think of sensor pixels, is as buckets which are filled by a rain of photons. At the same rate of rain (light level) the buckets will fill at the same rate, but the larger buckets will contain more water (photons). Since noise is essentially a constant regardless of pixel size, the larger pixel will have more signal, giving a better signal/noise ratio.

There have been improvements in sensors, allowing lower noise, but as these can apply to both large and small sensors, the advantage remains with the larger pixel.

Another advantage is in the 'blowup factor'. In viewing or printing at similar sizes, the smaller sensor cameras image has to be magnified 2 or three times as much as does an APS-C image.

brian


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Old Dec 21, 2007, 6:15 AM   #6
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mogumbo wrote:
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... Lens C set at aperture D ...
But to keep the same field of view, the larger sensor will need a physically longer focal length. That means to keep the same f/stop, it will have a larger physical aperature. More photons will come through a larger hole.

Also with larger sensors, there will be a smaller percentage of wasted space between each "pixel", or a smaller area where photons land uselessly. Or more space between each "pixel" to reduce interference (purple fringing?) - yet another trade off.
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