OK guys, so I was asked a very good question:

Why does the 400D exibit more noise than the 350D?

Here is my answer tell me who agrees and who doesn't... (shooting in RAW)

You'll see noise much more in the blue and green tones of an image, more so than the red. If you look at the images you shoot you should notice more noise in the areas that are very colourful or those that are one bright colour (like a blue sky).

The 350D is better at handling noise than the 400D and the 300D is even better than the 350D.
The reason for this is that as the MP of a sensor increases so does it's ability to fit more detail into the pixels of the sensor. What this means in short is that the same sized pixels on a sensor in a 300D(6MP), 350D(8MP) and 400D(10MP) have to fit more light in and hence more noise is fitted in too.

You should also notice this with cellphone cameras too. If you take an old VGA camera phone (NOKIA) and then take one of their new 2MP camera phone and take the same image and compare them on the PC. You will see that the new image is more 'in focus' because is can fit more detail in as that's how it's programed. But the VGA picture will have much much less noise in it (actually nearly none) as the sensor is the same size in each phone, they've just been programmed differently.

What happens in ISO 100 and ISO 400 the sensor is programmed to read the same amount of light but at different speeds. So if you are shown 20 cards and asked to remember all of them in 2 mins (ISO100), that's easy and you'll do a good job of it. But if you are shown those same 20 cards and asked to memorise them in 30 secs (ISO400) you'll only be able to remember some of the cards. That's why there is more noise at ISO 400 - the camera is trying to 'fill in the blanks'.

Now if you think of the 10MP sensor of the 400D and the 8MP sensor (exactly the same size sensors but prgrammed differently) this is what is happening at each ISO. Let's think in ISO400 seeing as that's what we're talking about.
Using the cards again. Lets say the 8MP is worth 80 cards and the 10MP is worth 100 cards (in detail).
If you use the same settings (shutter and aperture) and apply the same rules as I discussed above, you can understand why the 10MP can pick up more detail than the 8MP.

So what is happening is the 10MP sensor sees 100 cards but can only remember 80 cards - takes a success of 80%.
However the 8MP can see 80 cards and remembers 70 cards - takes a success rate of 87.5%.
The gap is not big but the 10MP sensor has more 'gaps' the remember. So while you get all of the 80 cards that the 8MP can see (think of this as detail - and more detail means bigger prints) it also comes with more forgotten cards (think of this as noise).

I've attached photos in the following replies:

1st pic - 350D sensor seeing detail as 80 cards out of 100.
2nd pic - 400D sensor seeing detail as 100 cards out of 100.
3rd pic - 350D sensor seeing detail as 80 cards but only remembering 70 and 'filling' in pink areas with noise.
4ht pic - 400D sensor seeing detail as 100 cards but only remembering 80 and 'filling' in pink areas with noise.

Does all of this make sense? Or am I just a pathological rebel?

So you can see the yellow is detail (MP size) and the pink is niose. In picture 4 there are more pink squares (pixels) hence more noise.

So are these (increased MP sizes) simply more detail and bigger print sizes at the sacrifice of noise (and ultimetly quality). I mean can you imagine how noisy these APS-C size sensors would be at 20MP?!

Granted you would have to print at like 2m x 2m to see it, but still it's there in the long run, on sceen...

P.S. can one of the MODS take the white highlighting out of my post?
There's no 'No Fill' selection... And the 'CLEAR' button doesn't work...

Got it

A well argued  case and after a couple of drinks I might be inclined to go with it, but then again after a couple of drinks I go along with most things    

I've lost count of the number of times someone explained the secret of the universe to me, I believed them all, trouble is it ends with me sleeping on the beach under Brighton pier  while they walk away saying well that was fun   

Trouble is these schemes were always based on some supposition to start with.  Bit like the lost motorist who stops to ask directions - back comes the reply " Well I wouldn't start from here" and the guide then explains the directions from somewhere else to their destination. 

So if I substitute your pixels for bricks what you are telling me is there are more bricks in a ten foot high wall than an eight foot high wall.  Yep,  suppose there is, but the number of bricks per square foot is still the same. Your supposition is that because the picture is bigger from a 400D it will have more noise than the smaller picture from a 350D . But this is not the case, if both were printed at 300 pixels per inch the noise would just be bigger or is that what you mean by more?  

But this is your second supposition because in reality the 400D does not have more noise than the 350D when measured by any scientific means. At all ISO's the 400D outperforms the 350D in noise suppression. 

Ah well it is a good argument - I'll use it to explain why I don't shoot with  Hassy H1D 
 
 

Nah not that the noise would be bigger but that there would be more but due to the increased detail it would look like there is less until you could blow the pictures up to the 'same' proportions as the 'sensor' MP differs.

I'll just take 2 unaltered pictures and go and count the noise dots!!!
See you when I'm 85 LOL...

Quote - So if I substitute your pixels for bricks what you are telling me is there are more bricks in a ten foot high wall than an eight foot high wall.  Yep,  suppose there is, but the number of bricks per square foot is still the same.

As I read it, the number of "bricks" is not the same because the size of the wall stays constant. That's the area of the sensor array. More pixels means you are building a wall the same size, but with smaller bricks (the individual sensor elements). And the smaller you make the bricks in a wall, the more apparent the mortar filling the gaps will become when you look at that wall. It may not be a very scientific - or even accurate - analogy, but it's the one I'll try out if someone ever asks me to explain this stuff. :-D Besides, in any comparison of image quality possible changes in noise reduction methods built into the camera would have to be taken into account, not just the MP difference. And I soooo have no idea how much that was changed from the 350D to the 400D. :-D

My wall may not be the same as your wall -  the sensor size is the same and the number of light receptors is more on one than the other but I like to deal with something I can hold in my hand and that is a print.
A print made from the 400D printed at the same resolution as a print made from the 350D would be larger.

It would be larger and have more noise - HA HA pointing finger like Sherlock Holmes

I like the bricks and mortar thing :)