The calibration is a problem, which has prevented me from going out and getting a cheap digital for just this purpose. I have quite a nice handheld meter for incident readings (a fair amount more than $10 BTW). Not very easy when using a macro setting on the camera lens for exactly the reasons Rork brings up; still need to do arithmetic in my head. But folks DO use poloroid backs for this... Maybe does need to be built into the camera? Good points, Rork.