Metabones speedbooster: is there any reason to get full frame now?

If it has the same FOV, same DOF, and collects the same amount of light,  how does it become "crappy?"

There is a huge difference where you put the optics.

It's no better than than the 135/2 on FF but it's also not worse.   There is not much size difference between a 135/2 and an 85mm/1.2 but there is a huge size difference between a 50mm f/1.8 + compressor and a retrofocus 35mm f/1.2.

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Erik