Is a speedbooster just a reverse mounted teleconverter?

[Bronson Dugnutt]

Does my harebrained idea of adapting Pentax 67 lenses to a 35mm body stand any chance of success?

 

[abruzzi]

I don’t know what a speedbooster is, but Pentax sold an adapter to mount 67 lenses on Pentax K mount.

 

[BobD]

Does my harebrained idea of adapting Pentax 67 lenses to a 35mm body stand any chance of success?

Yes, just use the Pentax adapter made for that purpose but no speed will be boosted.

 

[LiamG]

A speedbooster is, in essence, a kind of reverse teleconverter- its a converging group of lenses, rather than a diverging group, like a teleconverter. That doesn't mean that a speedbooster is literally a reversed teleconverter, or that you can achieve the same effect by mounting a teleconverter backwards, if that's what you're asking. It's certainly theoretically possible to design a lens group which adapts medium format lenses to 35mm in the same way as a speedbooster is used for APS-C and MFT, but like others have noted, I don't think such a thing exists currently, and adapters for P67 lenses and others generally don't have an optical component, they just get the flange to film distance right and make the mounts compatible.

 

[Rowreidr]

Kipon Baveyes makes focal reducing/speedboosting adapters for medium format, including Pentax 67 to certain 35mm, mostly digital, mounts