Have you tried using the AMD graphics drivers provided by Lenovo? Using those drivers should resolve the brightness issue. The generic drivers that AMD provides from our website are not customised for your system, so all functionality (including adjustable brightness) may not be available.
Was my post helpful? If it was, please make sure to mark it as correct/helpful in order to assist others.
Thanks for your answer Matt,
unfortunately there are only drivers for Windows 8 for this Laptop. The brightness problem started even in 8.1, when I remember it correctly.
I tried them, too. But then I did not even have the right resolution as option. This works with the latest AMD drivers.