If you keep the sensor size fixed, then higher megapixels means smaller microlens size or smaller gap in between the lenses. So "everything being equal" doesn't apply here.
Higher mpx does not mean higher pixel density due to difference in sensor size.. but higher mpx against same sensor size means higher pixel density, but that also does not translate to better noise. because
1) The actual sensor size matters
2) Type of sensor matters
3) On board technology and picture processing matters
Note that pixel density should not be used as a predictor for image quality but instead as a parameter to help understand the sensor. But having said that, so far, all low pixel density sensor exhibits low noise at high iso due to the sensor being space futher apart and therefore improving the sensor sensitivity to light.
I am not good at these technicals but my understanding is, image noise are produced when signal are boost electronically. Not to be confused with detail loss from stray lights in small sensor or denser pixel per area. Larger sensor has the advantage of more light hitting the surface giving more energy signal (don know what to call). It will suffer from loss of detail rather than noise when density increases.
IMO, comparing into past model is not accurate because of technology. I have not seen it but I believe D40 might not be able to achieve 3200 low noise quality because of technology.
F30, LX3 and F200EXR, if you notice produce low noise images but at the expense of detail.
Foveon (for now) has no problem with sensor surface area but suffered from a technical problem of requiring much more light than Bayer bec of multi layers, thus generating more noise and lose of color in lower light.
lets have a common consensus, full frame Dslr = high end camera ?
if so .. of coz he has no problem with a higher ISO .. lolx ..
try using a 350D and see if there's any noise .. i would say tons of it .. vs a 50D .. same iso setting ... same subject ..
*sigh* i wish i can afford a 5DmII ..
Canon 50D| 28-75mm 2.8 : 50mm 1.8 : 85mm 1.8 | 580EX-II |
High end camera does not imply clean high ISO. It's a very very very silly thing nowadays for everyone to be fixated on high ISO when they would not normally shoot at high ISO anyway.
You're going to comapre an old and outdated 350D vs. a new technology 5dMkII? The differences in image quality will come from modern technical advances rather than "high end vs low end" thinking.
Also, if you actually look harder, you'd realize the 5DmkII gets "clean" high ISO at the expense of contrast and detail. Images become softer.