[wim]

You don't get more light on MFT at f1.4 than f2.8 on FF, though... And ISO can be set to whatever the situation asks for.

Trying to guess why MFT + this 12mm f1.4 lens appears so interesting to you, while you use a big DSLR with well spaced controls. Is it just the weight? It can't be the ergonomics.

Nor the size of the lens (as pointed out, it is not really small nor light).

 

Just as curiosity remark: Easy, cheap and light way to have 18mm on the 1D mk IV is to put a EF-S 10-18mm f4.5-5.6 IS STM on it.

 

You actually get the same amount of light per area unit, that is what aperture and iso are about, whether you use FF or APS-C, 4/3 or any other format.

 

Genrally speaking, and F/1.4 lens is useful becuase you can use a lower iso, and get less noise, or limit DoF, same reasons as always.

 

And weight is indeed about ergonomics.

 

Personally, I have a FF dslr, and a 4/3 camera, both with a few lenses. For exactly that reason: weight and size.  And 4/3 cameras have become really good.

 

Regards, Wim