On Sat, 15 Mar 2008, Mr Thant wrote:

On 15 Mar, 23:36, Tom Anderson wrote:

Well hang on, what? So a 10 MPx camera not only captures more
information than a 5 MPx one, but in fact so much more that when you
reduce the images to 5 MPx they still look better? This is a rum
proposition, most rum. How could this possibly be the case?

Google "Bayer filter", for one thing.

There is that. I knew about Bayer filters, but i'd sort of assumed that a
5 MPx camera had 20 million monochrome sensors with the filter over the
top, producing 5 million colour pixels. Which is not the case.

You're also assuming CCDs are perfect devices, without noise between
adjacent pixels, which absolutely is not the case.

I assume you mean that by downsampling, you average neighbouring pixels,
and so reduce the amount of noise. That would be true if it weren't for
the fact that CCD signal to noise ratio scales proportionally to the size
of the pixel (or its square root?); smaller pixels will suffer more noise
in the first place.

tom

--
All London roads are part of MY London Cycle Network. I'd like to see
some of them removed from the London Motor Network! -- Ben Jefferys

On 16 Mar, 13:37, Tom Anderson wrote:
I assume you mean that by downsampling, you average neighbouring pixels,
and so reduce the amount of noise.

The idea is that by downsampling you're no longer seeing the raw
noise, and the end result is more pleasing, even if technically the
image is numerically no less noisy.

That would be true if it weren't for
the fact that CCD signal to noise ratio scales proportionally to the size
of the pixel (or its square root?); smaller pixels will suffer more noise
in the first place.

I doubt there's such a simple mathematical formula in terms of
production sensors, because there are other factors beside pixel size
that influence noise levels (pre-amp and signal path design,
especially). It really depends whether this a theoretical or a
practical discussion.

U

--
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