Intel 4000 HD: weird auto bright/contrast in battery mode

My laptop is a newish Dell Inspiron 14z with Intel 4000 HD. Happy enough with the display when the laptop is plugged in, but when running on battery, some sort of weird auto-brightness/auto contrast kicks in and adjusts the display based NOT on ambient lighting, but rather what is actually showing on the laptop display. Bright objects make the display get brighter, darker objects make the screen darker.

This feature (?) annoys me, but I can't figure out how to turn it off. I've tried the Intel control panel in advance mode, and turned OFF the power conserving measures when on battery, but this doesn't help. I've also tried going to Control Panel > Power > Advanced Settings > Display and turned off anything likely looking there; no joy.

As a last gasp, I tried switching from the Intel driver to the standard Microsoft driver. This corrects the problem I described, but now the brightness is set to FULL and there's no way I can find to adjust it. It's TOO bright, so I've gone back to the Intel driver.

I see from other forums that I'm not the only one with this issue. Surely there must be some subtle setting I'm missing. Failing that, maybe there's a SIMPLE (even non-accelerated) Intel driver out there? One that doesn't try to be so extra-helpful?

If it makes a difference, the 14z is running Win 8, but a friend with the same computer running Win 7 is having the same troubles.

TL;DR: Gha! Intel! Jeesh!