I have been wanting something like this for ages (wide angle converter that's behind the lens)

Must have been one of the guys from the Fuji Forum....

I think the gist of the argument ( with whoever it was ) that rear focal reducers are covered by various patents and so is "impossible" ( cough ) because of the intellectual property issues.

My argument is that patents monetize ideas, as such if someone wanted to productize this, they could either just infringe and wait to be sued, or pay off the patent owner.   But its obviously possible, as you say, because that's the way photons work.

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