Electronic noise is related to the size of each sensor element, not the surface area of the sensor per se: larger sensor elements have less noise. There is also random electronic noise. Both of these impact the signal to noise ratio. Obviously fewer elements per surface area (i.e. larger sensor with lower pixel density) means larger sensor elements (larger pixels), whereas packing more pixels into a the same sensor means smaller pixels. It is not the surface area of the sensor so much as the surface area of the individual elements, thus the pixel density, that accounts for the better signal to noise ratio. Smaller, more densely packed sensor elements will not only have a lower signal to noise ratio, but there will also be the effect of random electrons causing "crosstalk" in the circuitry which causes noise. The denser and smaller the pixels, the more crosstalk in relation to the signal. So the little room packed with people versus the large room with only a few people IS a good analogy for at least one aspect of noise. Which is why the pro cameras tended to have larger sensors and lower pixel counts. The moral of the story of noise and low light performance is, "Don't be seduced by megapixel count".

Probably way oversimplified, but if each pixel generates noise than more pixels generally means more noise. Of course this is assuming that each pixel size generates the same level of noise and I think from previous discussions that we all feel that the larger pixels generate less. So, it appears to be a bit of a trade off when comparing pixel size and number of pixels. By comparison, a larger sensor with equal sized pixels as the smaller sensor should generate more noise. However, a larger sensor with larger pixels could actually generate less noise. Again, probably an oversimplification, but we also need to realize that with each new generation of sensors, the manufacturers get better at controlling the noise. Therefore, stating that one should stick with the lower Mega Pixel cameras to avoid noise is not really cut and dried, as there are other factors to consider. Good luck and good shooting to all.

It is sort of like packing a lot of people in a room. if you put 100 people in a small room there will be a lot of noise. Put the same number in a larger room, they will probably irritate each other less, and there will be less noise. Double the size of the room and the number of people you will get a lot of noise, but with the same hundred you started with, put them in the big room = less noise. The noise comes from them talking to each other. Same with electronic noise. Put a lot of electronic elements in a small space - they talk to each other = noise. Fewer elements in the same space, or same number in a larger space, = less noise.

Nearly all noise in a RAW image fille is thermal noise from the sensor.
It is relate to two things sensitivity (ISO) and sensor temperature.
Cool the sensor cold enough: no more thermal noise.

In electronics, noise is measured by signal-to-noise ratio.
There is no fundemental reason why higher MP sensors should
have more noise. HOWEVER, there may be an engineering reason:
inadequate cooling. This could cause slightly more noise when
shooting a high MP sensor at high frame rates.

Sensors located inside of cameras are difficult to cool adequately
But so far as I am aware, this has been a problem mainly in
lower-end cameras with "global shutter" sensors, not DSLRs
or high end mirrorless cameras with focal plane shutters.

Phase One makes 100MP backs for medium format film cameras
(real "pro" cameras, like the Miyama and Hasselblad).
These backs have cooling fans or thermoelectric
(Peltier effect) cooling systems.

The worst case for heat would be what al ot of peoople
are running out and buying:

* Mirroless camera with no mechanical shutter
* "Global shutter" sensor
* Fast frame rate
* EVF screen located inside the camera

Think about it: the camera is a small box, probably made
out of plastic. How you gonna cool it?

There are only four ways to move heat:
* Conduction
* Convection
* Radiation
* Active cooling (refrigeration)

None of them work well nside a small plasic box: How well
does plastic conduct heat, compared to say, copper? How's
the air flow throgh that box? If the sensor radiates heat, can
the IR light escape the box? And where you going to put a
refrigeration system?

The bottom of the sensor is covered with contacts--surface-mount
pads that get soldered to the board, so there's not even anywhere
to put a heat sink.

Phase One covers the entire back of the board in a heat ink. That's
not going to fit inside a camera.

Technology isn't about buzzwords--"The Cloud", "nanocrystals", etc---
it's about engineering. You can't get four pounds of crap into a
one-pound box no matter what the marketing department decides
to call it.

Maybe having a tiny, plastic camera wasn't really such a great idea
after all. Maybe that's one reason why Phase One's sensors are so
much superior to the ones the consumer camera companies are stuffing
inside of cameras.

Be careful what you wish for--because Nikon, Canon, Sony, etc. will
give it to you. Phase One, Leica, Hasselblad, Linhof, Sinar , Lockheed
Martin will not--they customers demand top performance, not top styling.