Mount Adaptor Setting

[jw432]

I use a number of manual focus 3rd party and vintage lenses I use on my x-e3.  I never set the Mount Adaptor Focal Length.  What does this setting do and would it affect my results?  I've not been unhappy with the results ... so why is this even a thing?  When would/should I use it?

[jerryy]

This stores the value you entered into the EXIF data. That can be useful if at some point in the future you want to remind yourself which focal length (lens) you used to get that image. Additionally, if your raw converter allows you to use custom lens profiles during the conversion process, this information already being in the data can save you some time by selecting the appropriate one for you. The lens profile can contain information regarding how much vignetting the lens has for a given f-stop, how flat or fish-eyed a lens is, etc. Many raw converters have some generic profiles they use for lens that cover a wide variety of settings. 

It does no harm to leave it empty, filling it in and remembering to use may give you some extra options.