Exposure value in tonemapping step is a constant

The exposure value for the tonemapping shader is hard-coded to always be 4. There are a few ways to automatically adjust this value to simulate the behavior of the human eye. Many graphic engines implement this behavior in some way so I would like to know why our engine doesn't do that at the moment. Does our tonemapping algorithm not support different exposure levels or has it simply not been implemented yet?

This is probably a question for @SamuelCho.