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http://www.ebay.com/itm/Mitakon-Len...ens_Adapters_Mounts_Tubes&hash=item4ad0bed903
lens turbo or focal reducer is a "speed booster" clone. Optics probably are not as good, and no apeture control, but it widens the lens back out a bit and adds a stop of light (compressing light photons or some stuff - but it does work) but it exists NOW and is 1/5 the price of speedbooster.
It works, I like it and kinda recommend it. So, one benefit of it is... lets say for my night scene I used my 50mm on GH4. I THEENK the crop makes that lens a 100mm? x2 ? Also that lens is not as sharp at 1.4 as it is at 2.8 or f4 and soft/fuzzy*. So, I can set lens at 2.8 (less fuzz, sharper), use the lens turbo and it widens back out a bit, and ALSO adds a stop of light (approx) back down to 1.8
* http://www.photozone.de/canon-eos/565-canon50f14apsc?start=1

Trying! No clue what I am doing, but pressing forward, and it is fun! Wallet is a black hole now though...I can tell you're really stepping up the production value on this one.