[Scythels]

There's no such thing as infinity.  There are the stars, millions of lightyears away, or distant mountains mere miles away.  At no iris size (=fl/f-no.) are they both in critical focus. 

 

If the adapter is micrometers too close to the sensor, the lens will be "backfocused" at "infinity," ie: focused further than you wish.  If the adapter is micrometers too far from the sensor, the lens will be "frontfocused" at "infinity," ie: focusing at a closer 'infinity' than you may desire. 

 

If the lens contains low dispersion glasses, the focal plane is temperature dependent as well.