M to L adapter effect on result?

[Genoweffa]

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19 minutes ago, Stuart Richardson said:

I think adapter makers usually slightly cheat towards focusing past infinity for the reason that Jaap explained...if you go slightly past infinity, the lens can still be focused spot on at infinity, whereas if the adapter is slightly too thick, then you will never be able to achieve infinity focus.....

Not sure re: 'cheating adapter'....when I put the same zeiss manual focus lens on my Canons w/o any adapters, the process of focusing @ infinity is the same, ie. have to go back a tiny bit for the spot on focus

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[earleygallery]

The camera is a box for the sensor or film. 
 

The lenses designed for said box will sit a determined distance from the sensor or film plane to operate correctly.

An adaptor simply enables you to fit another ‘non native’ lens by adding the required depth. 

You can’t fit a lens if the lens to plane needs to be less than it’s designed for e.g. an M lens to a R body unless you want macro only. 
 

Just to add, there are some adaptors which use additional elements to correct for the difference in focal plane, Canon FD/EOS for example. These might affect image quality due to the addition of extra lens elements. 

Edited March 8 by earleygallery

[BernardC]

22 hours ago, Stuart Richardson said:

Getting it exactly right is very tricky, especially with 60mp and 100% detail on long lenses wide open. This was not such an issue when the lens with the shallowest depth of field was a 90mm f2 that focused to 1m and on film. But with the APO lenses and high speed/high sharpness lenses we have now on extremely high resolutions, getting infinity focus perfect is extremely demanding on tolerances.

Also, Leica has been making rangefinder lenses for nearly a century now. Not all of these millions of lenses are perfectly calibrated. Better to make the adapter a few microns shorter so that they all focus to infinity on an SL.