pixel size, speed (ISO) and photon counts

[Ray]

in the deep shadows one must lose one stop of attainable S/N ratio for each halfing of pixel pitch

The Outback article does a good job at explaining the fundamentals. I find diagrams usually very helpful. But I'm still confused about the distinction between pixel pitch and pixel size. The D30 has a pixel pitch of 10 microns but a pixel size of only 5.25 microns (according to information from Michael). Wheras the 1Ds has a pixel pitch of 8.8 microns and a pixel size of (about) 8.8 microns, allowing for 97% coverage of the sensor (as opposed to 25% coverage for the D30).

It would seem to me such differences between pixel pitch and pixel size would have significant implications, one of them being, for example, that 4.5 1Ds megapixels seem to be about equal in performance to 6 10D megapixels at 100 ISO, if not slightly better. On the other hand, widely spaced pixels perhaps have the advantage of lower heat induced noise.

[BJL]

Thanks to Michael for the links to outbackphoto on pixels and such. The important point about the fundamental limitation on low light noise levels and how, in the deep shadows one must lose one stop of attainable S/N ratio for each halfing of pixel pitch leads me to a question, for someone like Samir:

How many photons per square micron do you get at a given aperture ratio, exposure time and subject brightness? For example, if we knew the answer for an 18% gray card in bright sunlight with f/16 and shutter speed 1/100 (the sunny 16 guideline for normal exposure with ISO 100 film), we would have some idea of how deep into the shadows a given pixel size will ever be able to go before there are too few photons for it to count, and hence could estimate the highest possible speed (ISO equivalence) for a given pixel size.