Close focus, non-lens adapter (M42-EOS), original and 100% crop
Close focus, lens adapter (M42-Nikkor/Nikkor-EOS), original and 100% crop
< Left: medium distance focus, non-lens adapter (M42-EOS), 100% crop
Right: medium distance focus, lens adapter (M42-Nikkor-EOS), 100% crop >
Conclusion
In addition to the enlarged "crop-factor" (the focal lenght of the lens seems to be longer with a corrective adapter) the extra element definitely has an influence on the lens performance:
Contrast: the contrast of the lens is considerably weakened by the corrective adapter.
Sharpness: the level of detail a lens can reproduce is lost a little, but the depth-of-field is a little wider with a normal adapter. So it's easier to miss a precise focus.
Distortion: I could not see any change in lens distortion.
Vignetting: It might be that edge darkening gets worse, but this effect is equalized by the increased crop factor. Bad corners are cut away.
Bokeh: I have the impression that the bokeh even becomes a little softer with the corrective adapter, but this can also be due to the narrower DoF.
All in all, a deterioration is visible but the adapted lens can still be used. The myths of horror stories as we can read them on the internet sometimes are clearly busted. If you can live with an increased crop factor (approx. by 1.3) and a small loss of lens speed, a corrective adapter can just be the solution for Nikon fans who want to try old non-Nikkor lenses on their DSLR.
Contrast: the contrast of the lens is considerably weakened by the corrective adapter.
Sharpness: the level of detail a lens can reproduce is lost a little, but the depth-of-field is a little wider with a normal adapter. So it's easier to miss a precise focus.
Distortion: I could not see any change in lens distortion.
Vignetting: It might be that edge darkening gets worse, but this effect is equalized by the increased crop factor. Bad corners are cut away.
Bokeh: I have the impression that the bokeh even becomes a little softer with the corrective adapter, but this can also be due to the narrower DoF.
All in all, a deterioration is visible but the adapted lens can still be used. The myths of horror stories as we can read them on the internet sometimes are clearly busted. If you can live with an increased crop factor (approx. by 1.3) and a small loss of lens speed, a corrective adapter can just be the solution for Nikon fans who want to try old non-Nikkor lenses on their DSLR.