Not quite that simple -- if I do the same comparison for the 135 and the 85, the 135mm f/2 replaces the 85mm f/1.2 on full frame (at about half the price) and the 200mm f/2.8 replaces the 135mm f/2. On full frame, lenses like the 85mm f/1.2, f/2.8 zooms, and the 200mm f/2 have no replacement on APS-C that provides the same dof and fov. So if you want shallow dof, you are much better off with FF.
In terms of the 400mm f/2.8 -- you have the 600mm f/4 which is about the same weight.
You will generally get more reach for less $ on APS-C though (you also have access to several very good and inexpensive wide angle choices)
I might be going stupid but I dont understand how can a one stop slower lens be the equivalent?
They are equivalent in fov and dof. They are not equivalent in terms of exposure because you need to bump ISO by a stop to get the same exposure. However, the larger full frame sensor will perform better at high ISO.
135 f/2 is not the same as a 200 f/2.8, a 85 f1.2 is not the same as a 135f2, a 400 2.8 is not the same as a 600/f4 -
No, they are not "the same" -- the only lens that is "the same" as the 135mm f/2 is the 135mm f/2. However, that lens will behave different on a full frame body vs a crop body. If you're looking for equivalence in terms of dof and fov, then my comments are correct.
please consider that there is a lot more to photography than a shallow dof - in these cases low light and speed.
The full frame sensor performs better at high ISO. You bump the ISO by one stop with a 200mm f/2.8 on full frame, and your dof, fov and noise level is comparable (maybe not exactly the same, but same ballpark) as your 135mm f/2 on a crop.
Shallow dof works against most photos in ff - macro, wildlife, landscapes - even street shots in poor light.
Yes, but if you want a lot of dof, why are you using an 85mm f/1.2 or a 300mm f/2.8 ?
I agree that a crop has the upper hand when you are at the other end of the aperture range.