This Sony projector has a dual function iris. One that can be used dynamically - adjusting frame by frame to the scene changes, or manually, where it can be used to lower overall brightness. (Do you have a smaller screen?)
That's not all, however. There are three reason's Sony's iris design is impressive. Note the Auto Limited mode. This works just like regular dynamic iris function (Auto Fill) but you can limit how open the iris gets. Thus, the home theater owner with a smaller 90 inch 1.3 gain screen, who might find the 900 calibrated lumens in Low Power mode to be a bit too bright, can still get the dynamic iris affect on black levels, while gaining the advantage of limiting overall brightness like a normal manual iris. Well done! (In fairness, this is nothing new for Sony ES series projectors.) For example, one could use the limit feature to limit the max brightness to something around 700 lumens if desired.
The third strength of this iris is its iris action. I don't know that it's smoother than the Epsons, for example, but both are pretty smooth, rarely noticeable except on unusual content, and things like flashing credits. As mentioned (in the black levels section) the Sony iris is, however, more sensitive in very dark scenes without any significant fairly bright areas. Because of that, the appearance of a pause icon is enough to have the iris open a bit on a very dark scene. That tells you something, but that doesn't make the iris action better or worse.
Bottom Line, the VPL-HW55ES projector has a flexible, well designed iris, and delivers deep blacks on dark scenes that rival anything at or around the price, and better than a number of more expensive projectors.