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Old Jan 17, 2005, 4:58 PM   #1
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If I use an 8mp camera and have high noise say at Iso 400 and I switch to a 5mp mode on the same camera still at Iso 400 will the pic be less noisy? What I am trying to detirmine is would it act as a 5mp camera with a large sensor. Hope this makes sense to someone.
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Old Jan 17, 2005, 6:31 PM   #2
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It takes the picture with all the pixels and resamples it down to 5Mp rather than disable some of the CCDs. So you wouldn't gain any noise advantage. Most digitals work that way – maybe someone can point out some that don't.
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Old Jan 18, 2005, 6:38 AM   #3
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You should see a reduction in noise by downsizing IF the camera "averages" adjacent pixels. It is possible the downsizing is done by throwing away some pixels in which case there will be no reduction in noise.

Do some experimenting - don't just believe everything you see on the web. Take a picture of a uniformly colored and lit subject (a wall) and measure the width (Standard Deviation) of the histogram. If the subject is truly uniform with no noise, it will be a single spike. Of course, no real subject will be completely uniform.

The reduction in noise measured by the Standard Deviation should be reduced by the square root of the ratio of pixel counts - 21% in this case. You will see very noticable decreases in noise when downsizing for the web - 640x480 should give an 81.5% reduction in noise when downsized from 8Mp. Since the subject and lighting will not be perfectly uniform, the actual reduction should be less than that.
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Old Jan 18, 2005, 10:19 AM   #4
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The fair way to make the comparison would be to take a picture at 8Mp and one at 5Mp. Take both to the size you want for output – but the same size in pixels for both images. Then compare noise.

I think what Bill is saying is that if you view them both at 100% you will see more noise in the larger image. Another way of saying it is that you will see more noise in an image if you blow it up. I can't disagree with that. But downsizing an image doesn't really reduce the noise regardless of whether it is done in the camera or in an image editor after the picture is taken.

So you can take the 8Mp image and reduce it to 5Mp in Photoshop and you will end up with about the same noise as having the camera reduce the image from 8Mp to 5Mp in the camera. Photoshop will average a resample if you set it to "bilinear" or give a weighted average of more pixels in "bicubic".

The only way to actually get lower noise would be if some of the pixels were selectively disabled, and I don't know of any cameras that do that.

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Old Jan 18, 2005, 2:56 PM   #5
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slipe wrote:
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The fair way to make the comparison would be to take a picture at 8Mp and one at 5Mp. Take both to the size you want for output – but the same size in pixels for both images. Then compare noise.
That is another fair test.

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I think what Bill is saying is that if you view them both at 100% you will see more noise in the larger image. Another way of saying it is that you will see more noise in an image if you blow it up. I can't disagree with that. But downsizing an image doesn't really reduce the noise regardless of whether it is done in the camera or in an image editor after the picture is taken.
No, I am saying the noise is decreased by downsizing, assuming averaging of pixels instead of simply throwing them away. The width of the histogram (Standard Deviantion) of a uniformly colored subject in uniform light is a measure of noise. That does decrease with downsizing.
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Old Jan 18, 2005, 4:35 PM   #6
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No, I am saying the noise is decreased by downsizing, assuming averaging of pixels instead of simply throwing them away. The width of the histogram (Standard Deviantion) of a uniformly colored subject in uniform light is a measure of noise. That does decrease with downsizing.
How does that differ from taking the 8Mp image and downsizing it to 5Mp in an image editor with an averaging resample mode?

I think everyone would agree you see less noise if you downsize the image or view it smaller. But that isn't the same as having the noise characteristics of thelower densitysensor.

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Old Jan 18, 2005, 6:47 PM   #7
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slipe wrote:
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... I think everyone would agree you see less noise if you downsize the image or view it smaller. But that isn't the same as having the noise characteristics of the lower density sensor.
True, but I wasn't saying anything about the source of the noise, or the comparison of noise between different cameras/sensors. Only that the noise is decreased with downsizing. Saying that downsizing decreases the noise in a measurable and predictable way. To put it bluntly, that you were wrong in your earlier statement:

slipe wrote:
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... But downsizing an image doesn't really reduce the noise regardless of whether it is done in the camera or in an image editor after the picture is taken.
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Old Jan 18, 2005, 10:20 PM   #8
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dwssas's query was:

What I am trying to detirmine is would it act as a 5mp camera with a large sensor.

The simple answer is NO. The camera takes the image with the same noise as the 8Mp sensor and downsizes it. You can gobbledygook the answer with all the square roots and Standard Deviations you want and the answer is still no, it doesn't act as a 5Mp camera with a large sensor.

There are scientific instruments that can deactivate sensors and give the effect of a smaller number of pixels in a larger sensor with resulting lower noise. It would have to make an 8Mp sensor into a 2Mp sensor though – at least with the limited number of devices I am familiar with. To the best of my knowledge all digital cameras use all the CCDs regardless of the resolution set.


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Old Jan 19, 2005, 6:45 AM   #9
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Indeed, that was second half of his question. It was the first half that I was answering. Your answer to the second half is correct.
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If I use an 8mp camera and have high noise say at Iso 400 and I switch to a 5mp mode on the same camera still at Iso 400 will the pic be less noisy? What I am trying to detirmine is would it act as a 5mp camera with a large sensor. Hope this makes sense to someone.
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