#Set X equal to Y Purchasing a second camera in the desired format is an alternative that may be worth pursuing depending on budget and goals. A functioning used camera may be available for the price of a new high quality adapter when the exist (or in some cases even a cheap low quality adapter).

###Rationale for this answer [meta] Because this general question serves as a basis for closing specific questions, it ought to include answers that cover the class of questions where given the specific details "What adapter should I buy?" is or may be an XY Problem.

I mean for the person interested in shooting old glass or buying an interesting lens, buying a compatible camera can be a viable solution or a reasonable extension of their unreasonable hobby. For example, a person with a Canon 5D Mark Whatever who picks up a Zeiss 75mm f/4.5 Biogon probably ought to consider a used press camera. Or more commonly, suggesting the purchase of an old Kmount camera to go with an old Kmount lens might provide a more reasonable answer than the detailed technical information contained in other answers here. And it might not. It really depends on the specifics of the question and the context for asking it.

Rationale for the rationale [meta meta]

Photography decisions are not just driven by technical considerations. Photography is an art. Decisions are also driven by cultural context and the desire of the photographer to express themselves. Technical assumptions produce a substantial fraction of XY problems.

End Remarks I figured I had better explain my answer here rather than fight a battle in the comments.

Set X equal to Y

Purchasing a second camera in the desired format is an alternative that may be worth pursuing depending on budget and goals. A functioning used camera may be available for the price of a new high quality adapter when the exist (or in some cases even a cheap low quality adapter).

Rationale for this answer [meta]

Because this general question serves as a basis for closing specific questions, it ought to include answers that cover the class of questions where given the specific details "What adapter should I buy?" is or may be an XY Problem.

I mean for the person interested in shooting old glass or buying an interesting lens, buying a compatible camera can be a viable solution or a reasonable extension of their unreasonable hobby. For example, a person with a Canon 5D Mark Whatever who picks up a Zeiss 75mm f/4.5 Biogon probably ought to consider a used press camera. Or more commonly, suggesting the purchase of an old Kmount camera to go with an old Kmount lens might provide a more reasonable answer than the detailed technical information contained in other answers here. And it might not. It really depends on the specifics of the question and the context for asking it.

Rationale for the rationale [meta meta]

Photography decisions are not just driven by technical considerations. Photography is an art. Decisions are also driven by cultural context and the desire of the photographer to express themselves. Technical assumptions produce a substantial fraction of XY problems.

End Remarks I figured I had better explain my answer here rather than fight a battle in the comments.