Hi vanzonhl, Roger talks about the processing that IMAX used which he was unhappy with. Once that was removed, he said the ALEXA images looked spectacular projected on the IMAX screen. We can't paste links here, but you can read more about this in our Skyfall article at the ARRI website

Because the dynamic range is less when you change the speed. ASA 400 1 stop lost in the highlights, 200 ASA 2 stops lost. 800 is the sweet spot. Especially since they need that extra range for outdoor high contrast lighting.

I think imax has to be shot on film.. there is just no other way to get shots like in MI:4 or TDK. As we can hear here you just cannot blow-up a 4K image and project it on a 800m² screen.

it can crush your dynamic range a bit, noise too i guess but I know what u mean, pretty negligible .. i would much prefer decent internal NDs given a choice though.

Why do they sound like we can't change the speed in-camera ?

How many IMAX DMR releases other than Skyfall have actually foregone the DMR processing?

Probably not many. IMAX strikes me as the type of organization that would foist DMR upon filmmakers. I have yet to see a single DMR release that didn't suffer from the conversion. That being said, "Skyfall" really looked good at 4K.