Hello,
Both have to check from terminal, depending on video graphics model what this command shows:
ls /sys/class/backlight/
Then, depending what backlight utility you installed, then it can be set to have a different steps values for increase or decrease:
https://wiki.archlinux.org/index.php/backlight
There is also a good way to provide information, so we can help you out without guessing, or sending you to the wiki. Here it is:
Once you gathered that information, let us know.