I have installed AOKP 4.3 ROM on my Xperia Z. When I need to take a fast picture, I just use auto ISO/Exposure...
When I wanna get better picture in low light, I just use a little bit tweaking
In low light, when I'm taking picture of some close object, I just put ISO on 100/200 (max), use flash and that's it! When I'm taking picture a little bit further (3/4 meters), I use 200/400 ISO, and use flash if I can (very big con of our device is bad flash). When I'm doing night portrait, just put ISO on 400 and turn on HDR, photos will look awsome believe me
Tonight I could upload some pictures from my Z, just to give you some clue.
P.S. When you're using front camera, turn on HDR you will get a much better photo quality