which is my point if u bothered reading ie the f4 lens acts just like a f2,8 lens.
You dont lose anything going from a 17-55f2.8 on a crop, to a 24-105L f4 on FF.
^^^
I think DP's point is that you spend allot more, but don't actually gain anything (noteworthy) with that setup.
yes, I think we all agree here. You don't gain much moving from a crop with good fast glass to a FF with slower glass.
You have to look at the inverse, a 24-70 2.8 on FF would need a 16-55mm f/1.8 to give the same DoF and low light performance. Well you can buy a 50mm f/1.8 prime fine, but not as flexible, at the wide end there is nothing out there.
It depends a lot on the lens, yes, but it's not down to whether the lens itself is good or not. The 135L autofocuses very well on the 5D, as does the 70-200 from the brief play I had with one, but the fast Canon primes, which are a lot of the appeal to their system, do end up highlighting the weak points of the 5D2 AF.

