Please bear in mind that Intel uses generic video drivers for your Intel® HD Graphics 4000. Some graphics features added to your graphics controller may or may not work if you are using generic drivers, that is why Intel recommends customized drivers from Lenovo.
You could try using Screen Brightness from Power Options.
I Tried to rool back the driver but it dd not work...
then I tried to download current driver from Lenovo Z500 web site but Win10 did not allow to install that driver....
Do you have any other ideas???