I am currently using ZenFone 5 now with android OS 5.0. Even though I've set my brightness to the lowest but it is still too bright for me, and it's battery capacity is low too... so is there any ways to decrease my screen brightness so that my phone won't die too fast... sorry for my bad english...
There are two problems you report. One is the excessive screen brightness even at the lowest level that your settings allow. The next is that you want to reduce the battery consumption due to this higher brightness.
If you want the first one but not worried about the second you can try screen filter apps. These will reduce the brightness as perceived by our eyes, but will not do any battery saving stuff. Please look at the apps similar to Screen Filter
On the other hand if you want power saving as well then probably you should have a rooted device that will allow the apps like Velis Auto Brightness to work.