Nikkor Z - disadvantage with extension tubes?

[Nikfuson]

After trying the 50/1.2S earlier I remember I made some test shots with an extension tube (Fotodiox 15mm) and noticed that the image seemed more bothered with spherical aberration than what I remember when I made similar shots with the an FTZ'd F-lens.
I don't have any direct image evidence so this discussion is more theoretical, and quite frankly not THAT important to me.
Came to think of the short register distance of the Z-system and am wondering if the use of Z-lenses with extension tubes deteriorates the image quality more than with the use of adapted F-lenses.
The relative extension with say a 15mm tube is less on the F- compared to the Z-system (assuming the use of the same focal length of course).
My thinking is that the use of extension tubes brings the focal plane of the Z-lenses "further" out of optimised focus compared to what will be experiences with a F-lens. 
Can some of you with optical knowledge please enlighten me on this issue?

[Birna Rørslett]

I did notice this effect when I first added extension to the 50mm f/1.8 S-line Nikkor. One does get the evidence of what I have said for many years, viz. extension does impact the optical quality in a negative manner. In particular, spherical aberration and sometimes chromatic aberration increase significantly. Extension is not "just air".

[rosko]

I would try first to obtain an image of the same ratio (of a same subject)with each system. I mean your 50mm S lens against any 50mm F lens + FTZ. So you can have an idea. make sure the F stop is the same.

That wouldn't be a perfect comparison as the lenses are not the same, except focal distance.

I get nice results with my Leica R 100mm apo macro elmarit with leica R/ Z adapter on my Z6 or modified F mount Leica 60mm macro elmarit + FTZ.

I want to ad that I don't need optical knowledge to tell whether a lens is good or not.

Francis.

[Erik Lund]

Unit focusing lenses will fare much better with extension on mirror less - actually with no difference in image quality compared to when used on a DSLR at same effective extension.It's the IF internal focusing elements that of course makes lenses optical corrections fall apart when introducing extension.

[Jack Dahlgren]

I did notice this effect when I first added extension to the 50mm f/1.8 S-line Nikkor. One does get the evidence of what I have said for many years, viz. extension does impact the optical quality in a negative manner. In particular, spherical aberration and sometimes chromatic aberration increase significantly. Extension is not "just air".

Maybe a better way to say it is that extension (and the opposite in cases like O lenses) can take the lens outside the range it is designed for. Even within designed range some are better close and some far.