I use it sometimes.
What i do is to crank up the cn2 factor until you see something, then tune it as you would do with a real reflex camera (shutter,f number, film iso). The other settings are kind of color correction, and the gamma that you have to correct in every texture, not in the lens shader.

this render is using it:


https://features.cgsociety.org/newgal...0602_large.jpg


In the maya documentation there is an article called "Camera- and Exposure Effects" that explain nicely how to use it. It is in the mental ray for maya architectural guide, in the mental ray section.