Function keys with external keyboard

"The only way is to disable system-wide (that includes the notebook's keyboard) the custom control keys, and tell OS X that you just want to use it as normal F1-F12/F16 keys.
Go to System Preferences > Keyboard. Check the box that says ""Use all F1, F2, etc. keys as standard function keys""

If you are using a Mac keyboard, there should be a Fn key. On the latest Mac wired slim aluminium keyboard, it is around the home, page up, page down area. On the wireless slim keyboard, it is to the bottom left."

"You can do that with Karabiner-Elements:

It allows you to configure the Fn keys per device. In this case, I set it to use the default keys when on the external keyboard:"

If you are running Windows 7 under Boot Camp you can change in the Boot Camp control panel in your windows session.

"If you're using an external keyboard for a mac, check the external keyboard for a f-key lock button. On my logitech it is on the escape key.

(I just spent half an hour figuring it out!)"

I'm using Corsair K95 RGB USB keyboard, I too was figuring out how to use the brightness control on my Mac and then when I used the F14 and F15 keys, it all surprisingly worked out.