Raymo, I'm not talking specifically about AP here. I'm talking about the differences between the absolute entry-level DSLRs and the pro models, which is what Steve and I were discussing. Surely we all agree that DSLRs and the comparisons between them are meaningless when compared to purpose-designed CCDs. We're talking about what you get for your money when you go beyond the absolute cheapest DSLRs on the market.
ISO invariance, to summarise, basically means that you can underexpose a shot by several stops and not lose any detail (how many stops largely depends on the bitrate). It's explained reasonably well here:
http://improvephotography.com/34818/iso-invariance/
Raymo I know you're a big fan of the entry-level Canons, but I'm gonna just go ahead and disagree with you on their superiority over the ISO invariant Nikons. There's simply no way they can compete with the likes of the D750, or even come close.