Lamp Auto Iris
When I worked on the Sanyo PLV-Z4 projector review (and the Panasonic PT-AE900u) review, much of what I do is freeze frames to compare various aspects. I also watch short clips of many DVDs and Hi-Def content. Over the same time period I do try to just sit and watch a movie or two (or three), for the experience.
A purpose of the Lamp Auto Iris is to improve black levels in darker scenes (where there is no pure whites or full intensity colors). It certainly works.
But, bottom line, I believe that in such scenes the background should not change like someone is turning down the dimmers on the room lighting.
As a result, I would recommend, for most, turning off the Auto Lamp Iris on the Sanyo Z4, even though that should take a slight toll on black levels and shadow details. I had reviewed the Panasonic before the Sanyo, and never notices a similar effect - although, I have since looked at the Panasonic again. It too has its issues with frame by frame adjustment.
So for now, I find the Auto Iris to be distracting during some movie watching on the Z4. That said, even with the Auto Iris feature turned off, the image quality is excellent, although not quite as good as on for darker scenes. As a result, I'll assume the Panasonic projector has an insignificant edge in black levels compared to the Sanyo with its lamp iris out of Auto mode.