kernel param : why acpi=off breaks screen resolution ?

[Bruno Costacurta <techie@costacurta.org>]

Hello,

on my laptop Acer (model 4750) I have to pass kernel parameter  
acpi=off to be able to setup screen brightness via the Fn key.
However the screen resolution is then downgrade (to 1024/640 from 1366/768).

Why ?
Which solution should I try ?

Note:
I'm using latest Squeeze 6.0.2.1.
I tried same thing with Ubuntu 11.04 : it works perfectly as FN keys  
works without any loss of screen resolution.

Bye,
Bruno