I know Canon says it's for when you want smaller files, but I wondered if it improved the signal to noise ratio as it isn't a crop, has anyone here done the experiment?
I don't know for sure but have heard it involved color subsampling, which effectively decreases chroma noise and at least improves the 'signal to perceived noise' ratio.
I'd guess there is also some pixel binning, which would increase S/N.
Of course all that is at the cost of resolution!
Anyone better informed, please?