After launching DayZ and going to gamma and brightness you'll notice you can change it. If you record some DayZ gameplay the recorded footage will be very dark. Increasing game/brightness in game won't help. To fix that you have to hold alt and press tab until you are back at DayZ tab (Don't release alt in this process.). Now if you record any footage of DayZ you will see it won't be dark, it will be like it is in game for you. But by doing this, you can't change the gamma/brightness in game anymore.