I've been reading a lot about HDR techniques and came across Alpha Blending compatibility. Searched even more and found this snippet from the Ogre Forums:
With source alpha blending, it sometimes looks ok provided the range differences aren't too high, but most other blend modes fail completely, particularly add. You really have to do your own separate composition in another fragment shader targetting a second texture and pulling the 'background' values in so you can do accurate blending, rather than render the transparent effects into the same original surface.
Why is alpha blending an issue when it comes to rendering HDR (values above 1.0) in 16FP FrameBuffers?