the sensor’s pixels and readout circuitry employ new technologies that reduce noise, which tends to increase as pixel size increases.
im not nativ english speaking but. that sounds wrong.. not?
more noise with larger photosites?
I believe the point they're making is that, though the noise per pixel generally goes down with larger sensors, the noise per unit of area generally goes up.
So, with your low megapickle large area per pixel sensor, at 100% resolution (pixel peeping) things will look cleaner, but there'll be more total noise in the image as a whole than with a high megapickle small area per pixel sensor.
I'm not sure I follow the "noise per unit of area" thing.
I guess we really have to think of this in the context of very low light levels and not as compared to our dslr sensors. Certainly, for the same level of illumination each of these 19 micron sensels would pick up more photons as compared to a 4.39 micron sensel (like the 7D), and have lower shot noise as compared to the smaller sensel. I'm assuming, (but don't know for sure) that you'd have the same amount of read noise for the two sensels, and hence a better signal to noise ratio for the bigger sensel.
However, if you're pushing to record lower and lower levels of light intensity, then maybe what they mean is "as you try to read lower light levels (and use larger sensels), the shot noise becomes important and with lower light levels the read noise also has a bigger impact on the total noise.
Like I said, I'd be interested to see what neuro and jrista have to say.
There are inverse factors at play. Read noise is initially caused by dark current flowing through the sensor (with secondary downstream contributors as well). With a larger pixel area we have a larger photodiode, which means more area for current flow. That increases the contribution to read noise. By how much I can't say...depends on the materials used for the sensor, doping, and a number of other factors. I don't have enough information to offer specific numbers.
On the flip side, the larger sensor area means exponentially greater signal. The 1D X has a 90,000+ electrons in full well capacity (FWC). Assuming a 7.2x larger sensor area and the same Q.E., full well capacity should be somewhere around 650,000 electrons FWC. So, even at the lowest signal levels, there should be a far greater potential charge, simply because there is so much physical area for photons to strike per pixel. Assuming the sensor has a greater Q.E. than the 1D X sensor, then the potential for
true sensitivity is even greater, however the FWC is fixed by area, so a higher sensitivity simply means the sensor saturates faster.
The interesting thing about dark current, the prime contributor to read noise at the time of readout, is that it doubles with every 10°C increase in temperature. Conversely, it halves with every 10°C drop in temperature. Assuming a "room temperature" sensor (~23°C), a 10° drop in temperature should improve read noise by a factor of two. Now, it is unlikely a sensor will operate at room temperature, their density and the amount of current used for readout will increase the temperature by a certain amount. Lets say normal usage increases the sensor temperature 10-20°. To get any real benefit, we would need to cool by at least 30° to double read noise performance. According to the specifications of scientific-grade sensors, which use peltier cooling on CCD sensors, by around -80°C dark current is ~200x lower than at normal operating temperatures. That is a drop of ~125°C, so the improvement in dark current is non-linear as you keep cooling (otherwise one would expect a drop of ~1000x in dark current.)
(Aside: For those who wish to test this fact, you can try it with night sky photography on a very cold night. Anyone who does night sky or aurora photography in the northern (or southern) latitudes, you probably know that while your camera's battery performance drops significantly at low (sub-zero) temepratures, your night sky photos have very little, almost no noise. That is all thanks to the fact that dark current is proportional to temperature.)
Dark current today is already mitigated by using CDS, or correlated double sampling, which samples the charge in each pixel when the sensor is reset, and subtracts that charge when the sensor is read for an exposure, effectively eliminating dark current. Analog per-pixel CDS circuitry seems to be a contributor to banding noise, however, which is what lead Sony to move to an on-die, column-parallel Digital CDS approach in Exmor. Regardless, it is possible Canon has developed significantly more efficient CDS circuitry, which, when combined with moderate active cooling to keep the sensor below room temperature, could produce some considerable gains in read noise performance.
That said, if Canon still uses high frequency off-die moderately parallel ADCs in DIGIC chips, I would still suspect the sensor still has banding noise problems. I guess the off-die DIGICs could be cooled as well, and/or the frequency of the ADCs lowered (which should actually be more than possible with a 2.4mp sensor), both of which should lower the banding noise contribution from A/D conversion.
However, if you're pushing to record lower and lower levels of light intensity, then maybe what they mean is "as you try to read lower light levels (and use larger sensels), the shot noise becomes important and with lower light levels the read noise also has a bigger impact on the total noise.
This is true...photon shot noise becomes a problem at higher ISOs (actually, photon shot noise is the primary cause of noise at high ISO...increasing ISO itself does not actually contribute more noise). Nowever, the ratio of signal to read noise is MUCH smaller as well, which is why reducing dark current in the sensor is important. By reducing dark current, you increase efficiency, which supports a higher Q.E., which means that a greater percentage of photons incident on the photodiode itself actually free and electron. By reducing electron contribution to the photodiode from dark current, you increase "true sensitivity", thus making higher ISO settings more effective, with less noise. Combine that with a larger pixel area, and for any given unit of time, SNR should be much higher than with any current Canon sensor, at all signal levels.
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