I also found this
"Autodimming: this feature was put in by sony to reduce clouds by dimming the backlight in black scenes or mostly black scenes. the annoying thing about it is that it can't be turned off, so the backlight fluctuations do occur in movies with particularly dark scenes where most of the screen is black such as the credits at the end of the movie. it does get really annoying at times, for instance, when the credits were rolling with only a few names at the beginning, the backlight dims because it detects a mostly black screen. but then when more white texts and words rolled in, the backlight brightens up again because the screen wasn't mostly black anymore.
So you can imagine that it is even more annoying when this happens in the middle of the movie because when the backlight dims, it dims everything else on the screen such as colors and shadow detail. it is pretty subtle though and it doesn't dim all the time (only in very dark and mostly black movie scenes), but when it does, it is annoying as hell. I found a way to disable it, two ways actually. One is by turning ON the light sensor, but this only works in a completely dark room. so if the sensor only sees the dark, it keeps the backlight at the lowest level and never brightens. The other way is to just turn the ACE setting to HIGH which lowers the backlight somewhat(I just upped the backlight 1 notch and it evens out) but it stopped the autodimming."