One feature that Sony Mobile’s flagship Xperia Z lacks over other Xperia models such as the Xperia ZL and Xperia ZR is a physical camera button. However, xda-developer member black97one recently created a mod that allows you to use the volume down key to take photos. The mod also allows you to use the key to take pictures in fast capture mode from standby.
You can find the mod and installation instructions here. A recent update allows the mod to be used with the latest firmware build (10.1.1.A.1.307) as well as the leaked Android 4.2.2 firmware. See the mod in action below.