There are some really clever techniques out there for recovering the imaginary parts actually, at least in a similar domain (transmission electron microscopy), but they're still somewhat new.
Can't find a good holistic reference off-hand, but you're looking at focal series reconstruction (or, if you're using specialised equipment, holography) to recover phase information of your wave. That can then be used to reconstruct transfer functions and the like. [This isn't something I do personally so I don't want to go into too much detail in case I explain something incorrectly!]
The complex PSF isn't important for camera applications because the complex field carries the transparency information. I don't want to see transparent things with my camera.
