Guten Tag Jossie,
This is my take on the CMOS sensor design:
With the CMOS designed sensors there is a built-in amplifier under each photosite. The properties of this amplifier might be different (amplification increased or decreased), along with the integration/sampling rate. With amplification, there is always a noise-power relationship. Increased power typically hurts the signal-to-noise ratio. So, perhaps the result of a photon hit is more likely to be read out by the sensor/amplifier setup when its values are tweaked by higher ISO, which comes with a penalty of poorer noise figure, and/or the integration/sampling rate could be changed (I’m not sure how the latter item plays out). It’s been a long while since I looked at any of this kind of stuff, so don’t write it down.
As far as the difference between CMOS and CCD sensors, I found this:
Now I have to move into the realm of conjecture instead of just bad memory. Maybe the Foveon needs higher amplification and/or different sampling schemes from the start, due to its layering structure versus the matrix of Bayer. I don’t know any Sigma people, so this is total conjecture.
Here is another article that speaks to the Foveon sensor, but I haven’t decoded it yet, except to say that maybe it implies that the simpleton explanation for the ISO differences given in my previous couple paragraphs may fall short :
The latter link is an eye-waterer.
Sigma is thought to have much improved noise figures in the newer Quattro line. Another article I read implies that the noise is not really higher on the Foveon, but the Bayer scheme “averages it out” by interpolation. Is the better noise performance of the later Sigma models partially the result of averaging the stuff out in firmware? Or is it physical sensor change that improves the picture, or both? This may explain why Sigma people want you to use their software for post processing. The noise may not really exist.
IMO - The Foveon color is unbeatable, but maybe it needs more post processing to bring it out.