The magic of dynamic irises have dramatically improved the overall quality of home theater projectors. We've reached the point where most of the under $10,000 home theater projectors now have a dynamic iris to affect black level performance. Only some of the least expensive projectors and a few of the most expensive, now lack a dynamic iris. In other words, it's a pretty standard feature at this point.
Let's explore the purpose and the effect of a dynamic iris, on the projected image, and your viewing experience.
Since projectors cannot actually produce black, only very dark grays (varying based on the projector's quality), intended blacks on the screen are actually that dark gray. A problem occurs when you are looking at a very dark scene. Without bright areas to cause your iris to close a bit, those blacks start looking mighty gray. When you have a dynamic iris working, on those very dark scenes it shuts down, blocking most of the light. That makes the blacks a lot darker, but slightly brighter areas still remain pretty bright. It's a real plus. If however, your dark scene has some very bright areas, if the iris is to close down a good bit, those whites and near whites are going to get darker too. That means a bright white in an otherwise dark scene, won't be as bright as white on a normal or bright scene. The point is, you are changing the way things are supposed to be.
That's all just fine. The best thing about dynamic irises, is that they are the most effective on those really dark scenes, when you most need blacks to be blacker. On other, mostly dark scenes that have some very bright areas, the eye is drawn to the light, so the blacker blacks are less critical.
The other aspect of dynamic irises that I discuss is the visibility of their action. If you are looking, it's not hard to spot their action. Of course they are idle - wide open, on bright and mostly bright scenes. When you get scene changes, though, from dark to light, or the other way around, the iris has to react to that - opening or closing. If they wait too long, they can be visible. In the case of one projector, after a change in scene brightness such as that, the iris takes seconds to react, then snaps open, or mostly closed very quickly. Very annoying. Some irises are relatively slow - they average out the change and can take a while perhaps even a second to complete an adjustment.
That can cause a yo-yo type effect when scene brightness fades in and out every few seconds (happens a lot on movie credits like Superman, Star Wars, but in real content too.) One thing I look for in particular is when a scene consists of a conversation in a fairly dark room, say one with paneled walls. A person in a conversation, say wearing a bright shirt, who moves around while talking, may cause the iris to open and close slightly as the amount of brightness varies. That can make the scene come off looking like: you, watching a guy in a bright shirt talk - while someone off camera is playing with the light dimmer on the wall.
The viewing experience: Please keep in mind, it's my job to look into these things for you. So, while you realize that dynamic iris action can be visible, try to remember that with most of the irises on these projectors you will rarely notice their action - unless you are looking for it. I'll let you know which ones are those more likely to be bothersome. There are only a few with issues sufficient that I recommend turning the dynamic iris off.
I spend a lot of time watching movies with dynamic irises engaged. The bottom line: While there might be a occasional noticing of iris action, with most projectors that's a small price to pay for a significant improvement in black levels.