Bill, re: the Canon, at some point, Canon changed their ISO calculation method from the older standard 'saturation-based' (1/2 stop headroom) to something similar to 'Standard Output Sensitivity', or maybe 'REI', neither of which promise much headroom, if any. So the "headroom" on a 5D could be
next to nothing and photographers, accustomed to that little bit of extra headroom traditionally afforded by the older ISO method ISO = 78/Hsat, could take a while to become accustomed to the world of 'no headroom'.
Doug Kerr goes on about it at some length here:
http://dougkerr.net/Pumpkin/articles/SOS_REI.pdf.