Originally posted by Pentaxor ...4/3 system also can also use the same alibi that APS-C aficionados are using. can we then say why do I need APS-C?
Of course, that's true. Ultimately we could end up arguing that no-one needs a bigger sensor than that of a camera phone, so there are limits on how far one would want to take this argument. And no one is arguing the optical characteristics of FF re DOF, let alone stuff like the fact that FF bodies are usually bigger and built to pro levels of quality, which are also desirable attributes in their own right.
But as you step down to 4/3, at least for the moment, you do seem to notice a bigger relative decline in sensor IQ across all of the IQ parameters, not just high ISO sensitivity, eg something like this:
But having said that, there are many APS-C bodies that perform about the level of a 4/3 system like the GF1, so 4/3 performance overlaps APS-C to an extent, in the same way that APS-C performance overlaps FF. (And some FF performance overlaps MF too).
Which once again this shows how sensor performance is influenced by the march of technology as much as the simple relative physical size of the sensor.
And how complex camera buying decisions have become. FF used to be a guarantee of improved IQ across the board, I guess, but I don't see how one can say it is nowadays.