I recently bought a new Lenovo G505s laptop, with Windows 8 natively installed because I couldn't find anything with Windows 7.
Now, I configured the UEFI BIOS so it could accept Windows 7.
After downgraded to Windows 7, I have found a compatible graphics card driver, but now whenever I enable the driver it starts up with a really dim screen it is as though all the way down to the lowest brightness, and I can't modify it even in the AMD driver configuration settings.
A community user found the method below works for him.
1) Uninstall all AMD drivers and drivers, including the chipset driver
4) When it prompts to reboot, please proceed to reboot.
At this stage if you look in the Device Manager you should have both the integrated graphics and the discrete graphics card (if you have one) listed as AMD Radeon XXXXX (not as Standard VGA)
Try out the brightness keys if they work.
Note: Another method you may want to try is re-installing Lenovo Energy Management (can be found in support.lenovo.com) which has the driver supporting the hotkeys on top of the keyboards such as brightness and volume keys