newbie to mark III 1ds terminology

Please help a newbie in this:

the specs of the camera mention that the iso levels of the camera can be expanded. What does that mean? Is that a firmware change? A hardware change? If it can be expanded, why isn't it automatically expanded? Is there a reason why someone would NOT want to have the option of the expanded range of ISO?

thanks for any clarifications

CTB

It's the same with the 20D and whatnot - through a custom function you can "expand" the ISO range from 100-1600 (or whatever) to 100-3200 or 50-3200.

3200 will appear as "H" and 50 is "L" (I think, 20D doesn't do 50) to designate that they're not "true" ISOs and are pushed through software (slight reduction in dynamic range, iirc).