While the gamma has different objects on the screen varying in brightness between the two images, and, the Mitsubishi image appears a little brighter overall, look to the shades. The Mitsubishi's shades appear less black, a bit washed out. The HC4900's blacks just don't have a rich inky black feel. The Optoma definitely has the advantage in black levels. The difference is definitely greater than any exposure difference, so I have to say that the Optoma has a definite advantage.
Again, you can see a real difference in the dynamics caused by different black level performance. This time the Sony has the advantage over the Optoma, as the Sony's blacks look darker and richer.
And one more pair, this time, the Optoma compared to the Epson Home Cinema 1080 UB, which so far has provided the blackest blacks of anything under $4000.