Sorry, it's a long post, maybe too long.
In physics, the Carnot cycle is an ideal representation of a perfect engine having 100% efficency. It is a useful thermodynamic representation to explain energy conversion. In the real world, such a system does not exist.
Too many of you claim that a higher MP sensor does not have more noise than a lower MP one, all conditions being equal, and support this claim with mathematics, but that's not true in the real world. Some even claim that smaller photosites have less noise than bigger ones: now, that's the kind of claim that should make all of us invoke Santa, bigfoot and rainbow-pooping unicorns going on vacation together with a flying saucer. The real world behaves differently.
Saying bigger pixels let more light in is like saying cutting a 15 inch pizza into 6 slices instead of 8 gets you more pizza.
No. Your logic is flawed.
If the pizza represents the sensor, the number of slices correspond to the number of the pizza eaters, represented by the sensels, i.e. photosites, i.e. pixels. An eater who eats 1/6 of the pizza eats more than one eating 1/8 of it. An ideal 24 x 36 mm sensor being a single light sensitive unit is a one pixel sensor which, let's say, collects 1 billion photons at a given time unit and luminous intensity; it has the lowest resolution possible, but it would be capable of letting you know if even a bunch of photons have hit it or not with 100% certainty, i.e. with zero noise. Ideally, if you divide that single huge photosite into 1 million smaller photosites (1 MP sensor), each photosite receives IDEALLY 1000 photons under the same conditions; in the practice it's less than 1000 because of the wiring and the spacing between photosites which equally absorb the photons, but do not convert them into a useful signal, instead convert them into heat, which is detrimental. This 1 MP sensor has sufficient resolution to resolve enough detail for a very small print, and with today's tech you could probably use it at 204,800 ISO or more with very low noise (and before any of you reply that you can reduce the size of the image and therefore reduce noise and equally obtain the print, try exposing a 36 MP sensor at 204,800 ISO or higher if you can...).
Again, the same sensor with 20 MP exposed in the same conditions does not collect 50 photons per photosite, but MUCH LESS this time due to massive wiring and lots of wasted space between photosites, so you have a high resolution image, but with a lot of noise.
Actually, in my example with 1,000,000,000 total photons hitting a 24 x 36 mm sensor I think you'd have only random noise at 20 MP, but it was for the sake of explaining. I'm not talking quantum efficency at all here, it's just the number of photons you can effectively use I'm talking about. Moreover, we don't have a linear relationship between number of photons and noise, so it's not as if you have half of the photons per photosite you double the noise, the situation is worse in the real world.
The Canon 1Dx is 18 MP; in the Nikon D800, being 36 MP, each photosite collects LESS than one half of each of the Canon's photosites, that's why the 1Dx is much better in low light. And the D800 holds because of its superior sensor tech (let's face it, fortunately Canon's system is better as a whole), otherwise they wouldn't have made it 36 MP in the first place.
In the pizza analogy, the more you cut the pizza, the more breadcrumbs, morsels, atomies you produce, leaving the eaters with less and less pizza to eat to the point that, putting together all the slim slices of pizza eaten by all, they add up to not even a quarter of the original one. And, actually, a pie should be a better fitting analogy.
It's like having a 100 x 100 ft room all for yourself, 10,000 square feet is plenty of space. But if you want to accomodate 100 people inside it and offer them a bit of privacy, you have to build walls which eat space, not to mention furniture, so you end up with much less than 100 sq. ft for each dweller.
Still not convinced? OK, you may say "who are you to stand up and make such claims against my maths?", so let's look at what Canon's engineers have done, I bet they know more than me or anyone else on CR about silicon performance and noise. This is what I wrote in a previous post:
"There's a reason the 1Dx has the best (to my eye) IQ of all the DSLRs available to date (yes, better than any Nikon I think): its 18 MP FF sensor. And there's a reason why Canon developed a prototype sensor with photosites 7.5 times larger than the 1Dx: to capture quality video in candlelight (candledrkness sounds better, though). Don't know if you remember, but check these:http://www.canon.com/news/2013/mar04e.htmlhttp://petapixel.com/2013/09/13/canon-debuts-exciting-prototype-sensor-exceptional-low-light-capability/
And Sony? Compare the the 36 MP Alpha A7r(esolution) and the 12 MP A7s(ensitivity), then say again that more MP does not mean more noise if you dare. At base ISO maybe, but try going at 800 and beyond...
And should somebody dare claim again that smaller photosites means less noise as I've read too many times, remember Santa & Co. are watching us from their flying saucer... And again, at base ISO maybe, but what's the point of shooting 36 MP and then reduce resolution in post to lower the noise and make small prints or web sized images?
I'm going to spend the weekend with my son, so I'll be having a look at CR every now and then, but I'm not going to post, sorry. Have a nice weekend you all, too!