You can't talk about Signal/Noise in the sensor independantly from the whole system, as there is no Signal in the sensor considered separately.
I beg to differ.
The sensor receives and converts a "signal" regardless of whether you have a lens cap, a lens or an AA/UV/IR filter.
You can talk about Noise generated in the sensor, but Noise alone doesn't provide any information about the noise in your picture.
Yes, I -- and most of the regulars here -- are well aware of that.
However, when someone uses the term "noise" without any qualifiers in digital photography, he or she most likely means visible
noise, which is an indicator of the signal/noise ratio.
While it's not technically precise, it's the generally accepted term.
I can rant and rave about how stupid the use of e.g. "pathetic" as a derogatory term is in today's English, but it doesn't mean that I'll get any sympathy.
The 'generally accepted knowledge' is that under the same illumination, a larger photosite will receive more photons and have a better Signal/Noise.
I agree: the Signal will be higher while the Noise will have little variation.
So far, so good.
I would even say that smaller photosites have actually LESS Noise generated in the sensor (more about noise here) and this is exactly why I consider that saying "a bigger pixel will have less noise" is not true... (an oversimplification, at best).
Well, considering that a pixel only has a physical size on a screen, it's not even an oversimplification, it's a meaningless statement.
However, given the common usage of "noise" I mention above, saying "a bigger photosite will, in the same process technology and generation, have less noise ((compared to signal))" is quite reasonable.
As I said, there is little meaning in comparing different systems without taking into account the optics (current lenses and/or optical limitations implied by the system).
I will make a comparison: people now understand that resolution cannot be determined by Sensor's definition (MP) alone and that the lens has to be factored in. The big difference here is that the theorical resolution based on sensor's definition has a meaning... whereas the Signal/Noise considered independantly for the sensor alone has NO meaning at all.
Now, about that 'Same ISO comparison bias': people tend to compare Signal/Noise under a same illumination and say "everything else being equal". What this really mean is considering an Olympus 150mm f/2.0 on '4/3', a Nikon 200mm f/2.0 on DX, a 300mm f/2.0 on 24x36 (huh? where is this one???) and a 450mm f/2.0 on 645!!!
That "everything else being equal"(ISO/Illumination) is in reality the most absurd way to compare things: weight, price, Depth Of Field and even existence of products is not even comparable due to optics consideration.
You're missing my point.
Take two cameras with sensors from the same process technology and generation, with the same noise reduction system, but with different photosite sizes, from the same sensor manufacturer.
Use the same
lenses, subject matter and conditions.
If you don't worry about comparing possibly different noise reduction systems, you should be able to find something. I suggest the following cameras:
- Nikon D1H
- Nikon D1X