but the lens works so well on the 5D2.....
It is a case of the same lens being great on both Crop and FF and not having to buy a separate lens for both. I like how Canon can use FF lenses on it's crop cameras......
It's great if you use both "full-frame" and "crop-frame" cameras. However, for those of us who do not need *GASP* "full-frame", having to purchase "full-frame" lenses for our "crop-frame" cameras 'cause Canon neglected to cater to our needs, is bad economy: we pay for what we cannot even use.
Waste to use FF lenses on a crop frame Canon camera??? LOL Now I really have heard everything!!! One of he great strengths of the Canon crop-frame line is that it can use the entire family of EF-S and EF lenses! Moreover, issues like corner softness and vignetting either 'go away' or become far less acute. AND my 100-400mm becomes a 160-640mm equivalent!
Ah, the silly, silly things people complain about!
Also due to the pixel densities of current crop cameras, and so the potential for resolution, these cameras need the best glass possible. OK so you lose some of the outer image circle, but as Marauder says, that has its benefits too. Producing lenses of this quality is going to be relatively large and expensive whether they have an aps-c image circle or FF one.