If you read the Mental Ray documentation on the photographic exposure shader you can see that if the film_iso attribute is set to zero then the shader is in Arbitrary mode, then many attributes are ignored, like the camera_shutter and f_number for example.
I have created this AETemplate to add some logic in the Attribute Editor of the shader. The Template also group attributes logically and hides the more advance attributes.
Download the AETemplate here (View the readme.txt for installation)
EDIT (sept 04, 2009): I have update the script to force the automatic photon energy attributes of the physical sun shader to ON. This is usefull if you want to shoot photons from the physical sun.
It is also sugested in the documentation to set the rgb_unit_convertion value of the physical sun and sky to 0.318 when using the physical tonemapper.
To help you acheive this, you can download a modified render global script created by David Jonhson that will promp you with a choice of tonemapper to use when creating a physical sun/sky setup and set the appropriate rgb_unit_conversion value.
I have created a slightly modified version of David script.
David use a gamma value of 1 in both the simple and physical tonemapper and set the miDefaultFramebuffer at 0.45.
I prefer using a gamma value of 2.2 in both shader and leave the miDefaultFramebuffer value at 1.
This is purely a personal choice. I prefer controling the gamma in the tonemapper to avoid confusion.
My script also uses a promp dialog instead of a small window.
Again this is purely personal. I find this method simpler and more straightforward.
Both method are equaly good in my opinion, you can pick the one you feel more comfortable with.