Originally posted by fuent104
I don't think it's a matter of equivalency. I think the speedbooster reduces the entire image circle of the larger format to the size of the smaller format's sensor. This means you get the depth of field, field of view, etc., of the larger format.
Well, that is equivalency. It is a reverse teleconverter, essentially. I'm not saying it is a terrible idea, I'm all for any gadget. It is just less interesting that doing it with smaller formats where you can start out with a fast lens (like a f/1.4) and make it absurdly fast, where you've got a MUCH larger universe of full-frame lenses that can be used with your small-format camera, many resulting in a focal length/aperture combo not existing otherwise.
Using a .7x on most medium format lenses will get you something in the ballpark of a lens that already exists for that format, except now you're using an adapter with a big honking lens instead of a native lens. (So if you take a 55/2.8 medium format lens, you'd get something like a 38/2 on your full-frame with adapter.) But why not just buy a 35/1.8 native (which will be full AF, etc, using no adapter) and that would be very similar in all respects (and probably sharper). The one with the adapter will not magically look "more medium format" -- they'd be basically the same on the full-frame in terms of FOV, DoF, etc -- what matters is effective focal length and aperture. (I'm guessing the native lens would allow closer focus though.)
So maybe if you've already got those medium format lenses (or you can get them cheap somewhere), then have at it. But it won't be a big reason to go seek out those lenses unless you're actually going to shoot them on a medium format body also.