Hi all, I'm a long-time 7D user contemplating on upgrading to a 5D MkIII. It's known that FF users aren't happy with the 7D's IQ, but how big really is the difference?
The sensor is about twice as large, so in theory you get (slightly over) a stop of ISO. That is, all other things being equal, you'd expect the full frame sensor to have comparable performance at ISO 2000 as APS-C at ISO-800. In practice going from a Rebel XS to 5DII, I found I got about 2 stops.
You also get shallower depth of field for the same fov.
Is the difference in image quality similar to the IQ difference between a compact (e.g. G12) and an cropped body DSLR (e.g. 7D)? Or am I just expecting too much?
No, it won't be comparable to this for two reasons. One is that an APS-C sensor is about 8x larger than a 1/1.7" (e.g. Canon S100) point and shoot sensor. you're only bumping by a factor of 2.56 from APS-C to full frame.
The other is that the point and shoot has a mediocre zoom attached to it. With an APS-C camera, you aren't compelled need to make image quality compromises to cover a large zoom range or keep the lens weight down.
The effect is similar to making all your lenses a stop faster, and you have much more options for fast wide angle lenses.