if it does, I can’t see it.
here’s an attached example using Physical Units (on the right) and Unitless with a fairly high value (150000) on the left.
Notice how the sample background is color rich using a unitless scale whereas it’s practically invisible on the right.
The lighting on the two buildings seems the same though, at least to my eye.

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This worked pretty well.  Thanks for the suggestion, and thanks to everyone else for their input as well.

I have been reluctant to change the settings for the exposure control because the last time I screwed with them, the whole scene got blown out big time and I had to merge everything into a fresh scene to fix it.

If you are worried about messing up the scene, save another version of it or use the Hold function before you change the settings. The Unitless Physical Scale is just one parameter, it’s default is 1500, and you can always switch back to Physical units.
For an in depth explanation of Physical Scale, see the links to Zap’s blog in the above posts.

Setting the physical scale value to compensate your background image for exposure control is one way to go. But you will actually run into other problems when using this method combined with athmospheric effects for example, which also rely on the physical scale value to render correctly. Also if you mix photometric and “normal” lights in max…

So setting the output value of the background image would be the other way to accomplish right background exposure. You can actually calculate the proper output amount of the image in relation to the exposure value (EV) in photometric exposure control. You might even want to wire these parameters together with a float expression controller. This way your background output will automatically updat to the right amount when changing exposure settings. The only problem with this method is that your background image will still be influenced by the whitepoint setting in photomeric exposure control and you might notice some color shift in the image. Thats because the whitepoint (when expressed by temperature) will not be 1.0, 1.0, 1.0 but something like 1.1, 0.9, 095 (or anything like that). You could correct this to a certain amount by wrapping the background image to a color correction map which compensates for the whitepoint. But this is actually not very precise, though it will (after some testing) not be “visual” any more. Please refer to the thread on german-mentalray forum I posted in my first reply here if you are interested…

Hope someone might find this useful, as I have used this method quite a bit and didnet find any drawbacks by now....

Cheers ;)