Originally posted by Rondec I think a 55mm works pretty well on APS-C, as does the 77mm for portraits, but there is no doubt that full frame has an advantage when it comes to portraiture and subject isolation. It is not a miraculous thing -- particularly not in the 70 to 200mm range.
But the OP seemed to be talking about landscape photography, where I really don't see much of an advantage for full frame except when it comes to printing large.
Agreed, it's what I use today - either the 50mm 1.7 or 1.2, depending on how much of a rush I'm in (AF vs not), sometimes stopped down just a bit. I get results that are technically great, with the exception that it doesn't look like 85mm would in terms of facial distortion. I shoot the same subjects with the 70-200 in a slightly different way of course, so I see a difference. I really dig that Sigma 70-200!
For comparisons sake, here is a link that sums it up ( I think I've posted this before, but what the heck... )
http://www.stepheneastwood.com
The posing and framing with those two lenses/situations is quite different, and it's a practical impossibility for me to shoot the headshots I use the 50mm for @ 85mm (say with the 70-200) on APS-C, because I would need a ladder! Maybe with the 85mm on a FF I'd have to adjust a little bit, but at least within the realm of the possible. Let me explain...
I typically only have 5 or 10 minutes to shoot 8-10 portraits, often at the least opportune time & place during a wedding, so it's 'set exposure, pose subject, dial up focus, lay on the shutter while talking/directing'. Of the 10-20 shots per person, there is always a perfect shot. Since these are weddings there is no going back, and for consistency everyone has to be shot at the same time... so no ladder. These are shot from just above the subject; no double chins, shrinks away the body while still giving idea of how everyone was dressed, and it gives me great control of the background.
Is that too specific a use-case? Maybe. Probably for this discussion. But now that I have a crate of Pentax glass I'd rather they just gave me a dang FF, rather than try to unload most of it to pay for a CaNikon body. Or maybe a 645Z, but that's not ideal either.
Bringing this back to 'can you tell the difference?' - for landscapes, probably not. I'd rather shoot those big at f/64 anyway.
But do clients see a difference in their own image, or image of friends / family? I think so, even if subconsciously. But it may have more to do with things other than just DoF.