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I don't see how an f/4 lens on FF is "equivalent in effective light gathering" to an f/2.8 lens on 1.6x crop? Light is light, no?
I said that because a FF sensor has 2.6 times the area of a canon 1.6 crop. Since 2^1.38 = 2.6, that means that a FF sensor made with the very same technology should gather 1.38 f-stops more of light. In fact, one can crank up the ISO by 1.38 f-stops, getting the same noise at the image level (full picture) and turning the F4 lens "as bright" as a F2.5 lens is in a crop sensor, with the same noise. At the same time, that F4 lens is equivalent to 4/1.6 = F2.5 crop lens in terms of depth of field. The same reasoning may be applied to diffraction for a given resolution, concluding that both systems (Xmm F4 in FF compared to 1.6*Xmm F2.5 in crop) are totally equivalent. The end of the old FF-vs-crop wars!
Smaller sensors have the theoretical advantage of allowing more compact lenses. However, they lose shallow depth of field unless the lenses are made "abnormally" fast to compensate. And in such case, they are no longer compact. My first camera was a Olympus (C-8080) and thus I was interested in their developments, including 4:3 (a path I finally not opted for, though). Thanks to this, I was aware of one of their exotic developments, the "perfect zoom lens": the Olympus Zuiko ED 14-35 F2.0 (one of the best zooms ever made by humans):
This four thirds lens is equivalent (in FF terms) to a 28-70 F4 due to the 2.0 crop. However, it weights more than Canon 24-70 II F2.8. Yes, image quality is surely better (take a look at the review samples, totally uncorrected) but I'm sure that Canon would also be able to make such a miracle in their new 24-70 F4 if we were willing to carry 915 gr instead of the 600 gr it weights (and not to forget that the Zuiko starts at only 28mm-equiv, with no IS). Olympus had a huge delay placing this lens in the market, years after the initial announcement (it seems a very difficult design challenge). And this Zuiko "only" costs the same as the new 24-70 F2.8 II because it was released a few years ago (imagine it being released today, specially if made by Canon )
I personally prefer FF systems because, despite the cameras being a bit bulkier (the 6D is nearly a crop-size camera, though); same-quality, truly-optically-equivalent lenses seem not to be compact at all when paired with smaller sensors. And in FF there is less depth of field when required (most small sensor systems lack equivalent-fast lenses) or the ability to trade less depth of field in exchange for necessary scarce light. And there are smaller compact lenses too. Of course, I understand people who prefer smaller sensors and systems, each one has its advantages and uses, and not a single DSLR is pocketable at all...