Microlenses will collect light and focus it on the active sensing area of the sensel, but what about charge density, which limits the full well capacity of a sensel? The transistors in a CMOS will take up space and reduce the active sensing area of the sensel, leading to increased charge density in the CMOS as compared to CCD.
But Bill, we are talking about high-ISO performance - that was the original question. If you are forced into shooting in the high ISO regime, by definition you are not capturing much light/charge per pixel. So charge density and the full well capacity of the sensel are unconnected to this issue. These things matter when you have plenty of light per exposure, so you can shoot at low ISO, and can approach filling the FWC in the scene highlights.
I don't quite agree with point 3. For a given sensor size, pixel size is inversely proportional to the megapixel count. You can make the sensor larger, but at considerable expense.
Point 3 was simply saying that if I can increase the pixel size/area while holding the readout noise constant, I gain more signal-to-noise per pixel, and get better high ISO. That, for example, is one of the factors which gave the Kodak DCS 620/720x DSLRs the edge in high ISO sports shooting 10 years ago - while maintaining the same 15 electrons readout noise as the 9 micron CCDs of the time, their CCDs had bigger 13 micron pixels, with twice the surface area, and twice the flux accumulated per pixel per unit exposure time. So bigger CCD pixels are better for high ISO. But, I can also make CMOS pixels bigger, and they gain S/N in the same way. And making pixels bigger reduces the megapixel count for both CCD and CMOS, also in the same way. So pixel size matters in
absolute terms, but in this CCD vs. CMOS
comparative question, this factor cancels out.
small CCD sensor based P&S like S95 are <= 4 electrons, are they not ?
Yes. I think this may be because there is a secondary dependence of readout noise on readout speed. P&S cameras like the S95 are not designed for fast full-resolution frame rates (the S95 reaches 0.9 fps), their users have no great expectations in that area, so the readout can be a little more "leisurely" in order to keep the readout noise down a bit. Astronomical CCD cameras often exploit this by taking the readout speed way down, sometimes to the kHz regime rather than the MHz regime. We are always working at the limit of the S/N, so when you've waited 1 hour for an exposure to complete, you don't really mind waiting another minute for it to be read out, if it keeps the noise down.
I wish that MFDB manufacturers would introduce this as a selectable feature on their backs - a slower, lower noise readout option, for those who can wait a few seconds longer between shots.
did you try to compare CCD-based P&S w/ CMOS based FF dSLR 
? so is the difference because of CCD vs CMOS or because of other factors 
That comparison would be unfair, because of the vast differences in pixel size.
Hi what about Infra RED? I am asking because I have this CCD DSLR and I was thinking of removing the IR filter. From your argument the old adventages of CCD for visible spectrum no longer holds. Could you elaborate abut IR (for astro photo)
Good news for you J.: for IR, in principle neither CMOS nor CCD should have any advantage over the other. But, even with the IR-block filter removed, it does depend an awful lot on the transmission curves of the Bayer RGB filters over the pixels. In fact, the differences between various CCD Bayer transmissions can be larger than the differences between popular CCD and CMOS Bayer transmissions. This is clear from manufacturer datasheets. In MF CCD sensors, for example, the Dalsa CCDs I've seen transmit IR predominantly through the Red-filtered pixels (that's 1 in 4 pixels). In Kodak MF CCDs, the Green- and Blue-filtered pixels tend to recover in transmission as the IR band begins beyond 700nm, and beyond about 820nm, every pixel is transmitting equally; allowing shorter IR exposures, and giving monochromatic B&W IR images which in principle require no de-Bayering interpolation, which would allow perfect per-pixel sharpness.
All MFDBs are CCD (up to now), the P1 P25+ and P45+ are the world leaders in low light because of their ability to keep the sensor cool for long exposures, this is due to the great cooling system not the type of sensor used, the MFDBs are worst in their ISO performance because they lack any kind of noise reduction filter or circuit, this is a choice of the manufacturers due to the demand of the MF market for purity so that low ISO performance is maximum! The IQ of DBs is well beyond any modern DSLR even if they are 5 or 6 years old! The reason for CMOS sensors is mainly the low cost of production, but they do cost a lot more than CCDs to develop. I guess that the production volume of DSLRs compensates for the high development cost of CMOS. CCDs are considered sharper and with greater color than CMOS, I'm not aware which technology is better for high Iso, to compare that we should have a comparison of cameras with both technologies but the same NR system. Regards, Theodoros. www.fotometria.gr
Theodoros, there's quite a bit of confusion and mythology in your response. I'll take each in turn:
"the P1 P25+ and P45+ are the world leaders in low light because of their ability to keep the sensor cool for long exposures" - Here you are confusing long-exposure capability with high-ISO capability. They depend on different things. That Kodak DCS 720x I mentioned above, for example (I got one for a song recently) - excellent at high ISO, dreadful at long exposures. Nice interchangeable viewfinders though!
the MFDBs are worst in their ISO performance because they lack any kind of noise reduction filter or circuit - It's vital that people understand that noise reduction trickery is
not the reason why MFDBs cannot compete favourably with CMOS DSLRs on high ISO per-pixel performance. You can turn off the noise reduction completely, on say a Canon DSLR, and save a RAW file which has equal "purity" (to use your term). Apples with apples, the Canon still wins.
The IQ of DBs is well beyond any modern DSLR even if they are 5 or 6 years old! - At low ISO, yes. At high ISO, which is what we are talking about, no.
CCDs are considered sharper and with greater color than CMOS - Take the AA filter off a CMOS sensor and it is equally sharp as a CCD. Really, really. For that matter, put an AA filter over a CCD (I can, with my Kodak), and it is equally unsharp as a typical CMOS. As for color, that depends on the Bayer filters and the IR-blocking filter; there is no reason in principle, other than manufacturer design decisions on these filters, why the colour response of a CMOS unit cannot be the same as that of any given CCD.
What I'm saying is that you have to separate out what is intrinsic to the sensor (things like readout and dark noise) and things which are extrinsic (things like AA, IR-blocking, & Bayer filters layered on top of the sensor). Don't judge "CCD vs CMOS" on the basis of extrinsic things, which can be chopped and changed or easily redesigned. Kodak, for example, often offers the same intrinsic CCD sensor with different configurations of Bayer or no Bayer, IR-blocking coverglass or no coverglass.
I'm not aware which technology is better for high Iso, to compare that we should have a comparison of cameras with both technologies but the same NR system. - Or even better, with all NR turned off, and a RAW comparison with complete "purity". Well, that comparison has been done, over and over: it's an entire website called DxOMark! And I've done my own little version, comparing my MFDB to my Canon 5DII. Surprise, surprise, the Canon trounced the MFDB.
I still prefer the MFDB at low ISO and short exposures, and the handling of a MF camera.
P.S. I prefer to judge products with what they do for my photography, don't care much to analyze the technical origin of the solution given from manufacturers nor the scientific theory behind it for that matter.
That's a perfectly valid position to take, but then why did you depart from it, and feel you had to get involved in this thread?
Ray
