[UncleVanya]

Thanks for all replays, I didn`t realize that for such reducer to work the camera has to have different sensor distance than lens was designed for. That would explain why almost all adapters that I saw were for 4/3 cameras. But then again there is this Kodak patent that Mothballs pointed out. Too bad it will probably never see widespread adaption...
Front mounted adapter mentioned by promacjoe are nice to know about but that is not what I am looking for, I recently got a chance to shoot with Helios 40 and I was hoping to access the borders of the image circle where its “swirly” effect is most apparent.
I will have to save up for pentax full frame body then.

Not only distance but also size. The FF to smaller sensor size adaptation is more worthwhile the larger the delta. M4/3 is a 2x crop so you gain two stops and get wide angle coverage back. I'm curious what quality level the various no-name brand versions for K to m4/3 have I can't find any reputable versions from known companies.

[fuent104]

But aren't the equivalent lenses for full-frame already common and readily available? Medium format lenses are generally not as high resolving as full-frame and not as fast either. So you'd hopefully get that promised sharpness and speed boost, but you'd end up with a lens you probably already had for that system. What am I missing? (What would be the killer combo that you couldn't get otherwise with a native full-frame lens?)

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.

[UncleVanya]

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.

And you gain f/stops.

[vonBaloney]

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.

[raff89]

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.

I see that my question started a heated discussion so I'll join in.
I partially agree with vonBaloney, it is usually better to use native solution and from practical standpoint that would be definitive answer but photography is not all about practicality. There are other aspects to take in the consideration. My idea was based on a fact that there are so many great legacy lenses from before digital revolution and in some cases (like mine with Helios) you don't really have/can't afford native equivalent. I am sure that there are more examples and such adapter would enable people to get that special lens to be closer to its full potential. don't get me wrong, I myself was so relived when I bought proper macro lens and detached bulky bellow + lens combo but still it would be great to let my m42 lenses to spread their wings wider (sorry I got carried away).