AVS Forum Special Member
Join Date: May 2008
Location: San Francisco - East Bay area
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Auto iris is perfectly fine to use... opening and closing the iris should not affect gamut at all unless the projector is very strange. You NEVER should have Auto Iris activated when you calibrate a projector. If you are going to use Auto Iris (I like it quite a lot, especially Sony's implementation which is very very difficult to detect in operation... you might notice it 1 or 2 times during an average movie), when you calibrate, you should use a fixed iris opening in the middle of the range (roughly half-way between fully open and fully closed. When you are done calibrating, turn Auto Iris on and forget about it and enjoy your images. Auto Iris (especially Sony's implementation) closes the iris for darker scenes and opens the iris for lighter scenes. It will NOT make brighter scenes less bright unless you are using the "limited" version of Auto Iris present on some Sony models... the "limited" mode never fully closes nor never fully opens the iris. That could look dimmer than a fix and fully open iris... but the fixed and fully open iris will also have the worst black levels by far. I don't know how old the '60 projector is, but at long ago as 2006, Sony had a GREAT Auto Iris algorythm.
Auto Iris only messes up MEASURED gamma, It does NOT mess up instantaneous gamma of any sequence of frames. And it only messes-up measured gamma because it does change the iris opening as you change the % white... but you never see that happen while watching a movie. Let's say you set gamma to 2.2 with the iris half open. Then you change the iris to fully closed and measure gamma again... you're going to have the SAME RESPONSE from the projector with the iris fully closed only less light comes through the optical path. The relative "shape" of the gamma curve won't change. CALCULATED gamma will change because peak white is lower and dark black is lower and that will throw the calculated numbers off a little, but the projector will send the same amount of light from the RGB light engine regardless of what iris opening you use. The iris blocks more or less light as it opens and closes... the light engine in the projector is NOT changing what it does. So as far as the light engine is concerned, it is going to produce the SAME response with every different iris opening... only the brightness of the image on the screen changes. People always think what they measure with Auto Iris enabled is so messed up that Auto Iris is the devil incarnate... and it does LOOK diabolically awful if you try to calibrate grayscale with auto iris enabled. But that's an artifact of the measurement method --- it does NOT represent what the projector is doing internally.
Most projectors that have Auto Iris have black levels that are so high as to be consideraly more of a distraction from a satisfying viewing experience than using Auto Iris... however, there are some Auto Iris implementations that are just awful... take recent Brand E projectors for example... the iris motor makes so much noise when it operates, unless the projector is in a soundproof enclosure (or booth), you are going to EASILY hear it in operation unless the soundtrack is VERY loud. Secondly, their algorithm is so bad you can almost always see the light level on the screen changig. I don't know how Sony avoids that, but they do (from 2006 forward at least).
Calibrate in fixed iris mode with the iris at a middle opening setting. When you're done, turn Auto Iris on... I've never been disappointed by a Sony product with Auto Iris from RPTVs to the new 4K projector... they all work really well and they all produce much better black levels than is possible with any fixed opening except fully closed. Unforrtunately... fully closed limits peak white so much that bright images look dull. With Auto Iris in the "full range" or "wide range" or whatever they call it, mode, you get the darkest shadows and the birghtest whites the projector can produce.
"Movies is magic..." Van Dyke Parks
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