All sensors have one "native" iso speed. You can "push" it to higher
EI but you get more noise and less dynamic range. Withing a range of
EI though the difference is small. You can't "pull" it though, because
the highlights will get blown out. (At least without
some magickry like combining many superfast exposures like Kodak DSLR
does at ISO 6 - yes iso six).
Basically, the Sony sensor has native speed of 200 and the Canon
sensor has native speed of 100. Thus the Canon models have iso 100 as
the lowest, while the models using sony's chip have iso 200 as the
lowest.
Yes, there is "EI 50" setting at some Canons, but it is out of the
normal range, and it is actually _worse_ than the lowest native 100.
It's about the same as shooting at 100, overexposing one stop and
cutting back in the "development" of RAW processing. Gaining you
nothing.
thus, don't look for low isos. It ain't gonna happen soon. There are
low iso chips, but they don't have high sensitivity for pushing... The
medium format chips who have even iso 25 as their native are pretty
noisy at iso 400...
Good light!
fra
