The hardware keys for brightness control on my Lenovo T440s (2014) worked at a very coarse granularity. Hitting the brightness keys would cause it to switch between 4 levels of brightness. I found a solution here which worked for me.

Simply add this to the GRUB_CMDLINE_LINUX_DEFAULT line in /etc/default/grub :

GRUB_CMDLINE_LINUX_DEFAULT="acpi_backlight=vendor quiet splash"

and run

sudo update-grub

Reboot and you will be able to change the backlight brightness in finer steps.

Update: On a recent installation of 14.04.3 on a 2015 T440s, I didn’t have to do anything to get this to work. Seems like it was fixed upstream.