"Auto Dimming" was a feature introduced after Sony had such problems with clouding/mura with the XBR2 & XBR3. If the background is dark and you dim the back light, clouding is less likely to show up. With our XBR4, properly set up, it's only ever noticable during the credits. A non-issue, really. The only way to defeat it, that I'm aware of, is to put the TV in "Game Mode" which shuts off all of the extra processing, including auto-dimming.
Sony KDL-52XBR4,SD DVR530/630,Tosh HD-A2 HD-A30, Pana DMP-BD65, Pio VSX-1015,Energy C3,CC100,ESM-4,S8.2, Engenius WAP