A problem that is often encountered by the would-be ETTR shooter is knowing exactly how much he can move the histogram to the right before it clips. Relying on the camera histogram and/or clipping warnings (blinkies) can be very misleading because they are derived from a highly processed version of the photo. Similarly, the histogram of a shot of a grey card does not represent the real RAW capture, but rather the result of processing designed to render the capture as a medium grey in a finished version.
There are only a few ways to see a real RAW histogram; to know what your camera really captures before white balance, tone curves and other processing fundamentally change it. In the past we had the excellent Rawnalyze program, but unfortunately its author died a few years ago and it has not been updated for new cameras (if you can even find it). Now however Iliah Borg (who is well known in the Nikon community as a veteran creator of NEF RAW converters) has authored a new application for analyzing RAW files, Raw Digger. It is still in beta and can be downloaded at:
http://www.rawdigger.com/
Using the program one can, for instance, determine the exact highlight clipping point for your camera as encoded by the ACD, which is rarely at the 14 bit maximum (16,383). For the 5D2 it is 15,760 (before black point deduction). Once you know that, you can determine the value of the RAW capture according to a spot reading and thus know the number of stops of headroom between the spot meter calibration and clipping. For a fuller explanation and examples, see:
http://www.rawdigger.com …se/lightmeter-calibration
