Last weekend was my first time as DP on a film production. This was a short film done on 16mm but I've gotten so proficient on my 550D with ML, it made an excellent communication tool to communicate with the director. I used it to stage camera movements, set up framing, and check lighting. It was totally indispensable.
The awshit moment came when I got vastly different meter readings from the lightmeter we were provided (a little sekonic) from what I got from my DSLR. And the disparity was dramatic. After lighting the whole scene with my gaffer, the sekonic said we should shoot at 1.4, but the picture I saw on the 550D looked right at 2.8.
To make matters more complex, I used the new ISO exposure compensation to achieve ISO 500 - this matched the Kodak 500T we were shooting on (to say nothing of manual kelvin adjustments - doesn't exist on this camera without ML thanks to you).
With the whole crew waiting and the producer anxiously pointing out we were 2 hours behind schedule, knowing that with this old arriflex 16mm camera (no video tap) I wouldn't get the results back until days later, I made the risky judgement call to shoot at 2.8.
Would I ruin the whole picture? Should we hedge our bets and push process a half stop?
We just got the results back, and ... NO! Director says exposure is great. Even a tad bright in some places for a scene intended to be dim but nothing the immense latitude the film can't handle. I can't wan't to see it myself.