24-85mm 3.5-4.5 vs off-brand 24-70mm 2.8

I wasn't going to explain this to him, but then I realised I couldn't be bothered to waste my breath!

 

I mean, I am likely to buy a D600 if it comes, that's why I needed to talk about equivalence. I just want some time to decide. If I were to buy my current setup immediately after I made up my mind before, I would have gotten a totally different setup. More time to decide means less wrong choices. Sorry for not stating that if I'm not clear.

 

I will try one last time.

'Equivalence' is telling you what f-stop you need to set on a FX lens on a FX body to get the same DoF as a DX lens of the same focal length on a DX body. It does not change the fundamental properties of a lens - the maximum f-stops of the lenses are what they are because of the lens design and do not change with different formats. The differences are that FX lenses have a larger image circle than DX lenses so that they can adequately cover the larger format. As format size increases so a lens has to be stopped down further to give a particular DoF compared with a smaller format.

You do not need to use equivalence just because you might buy a FX body - you just need to understand that a DX lens will not cover the full FX format satisfactorily
but even if it did it would still have the same maximum f-stop as when used on a DX body.

'Equilivance' as you are using it is a means of determining what lens setting to reproduce DoF when using different formats - it is not some magic formula that can change the characteristics of a lens between formats and cannot be used to compare two lens in the way that you are doing.

Over and out.

 

You need to crop the DX lens anyway if you put it on a FX lens, that's why you need equivalence.