Comment 8 for bug 1724933

This appears to be quite an old issue with Optimus graphics. For me the workaround was to pass specific kernel parameters at boot time to inhibit acpi control of the backlight but allow intel_backlight control
I followed the instructions at https://nisheetlall.wordpress.com/2016/07/12/solved-brightness-fix-in-linux-for-optimus-laptops/ which worked first time for me. Your mileage may of course vary - it happens that I have exactly the same Lenovo Z570 laptop as the author of those instructions.