Ultimately, it has nothing to do with the spacer ring, since he no longer to change than the distance power, and the strength of the effect of this change will be significantly influenced by the focal length of the lens:
At about 50mm one has with the spacer ring a working distance of a few centimeters in front of the front lens, including one is partly at 0cm or less. That sounds like "great", but the magnification is thus not very big, because the focal length is too low.
The greater the focal length, the less influence the distance the near distance. In addition, the imaging quality of the lens is critical, whether as a Nahdistanzring worthwhile. I once tested a few common lenses, all in the full format, purely subjective rating!
Canon EF 100mm L: Virtually no optical loss noticeable but little gain in close range
Canon EF 24-105 L: some strong optical loss, especially in the corners, high gain at close range (at 105, it is almost a macro lens)
Canon EF 70-200 L II: no significant optical loss, good gain in close range to 100mm focal length
Canon EF 50mm 1.4: strong distortion especially at the edges, for me no practical use more ...
Canon EF 75-300mm III: low optical loss, good gain in close range to about 150mm.
Conclusion:
Ultimately, it depends strongly on the lens. Is this the "close-range" (which depending on the lens with 40cm or nearly 2m is) corrected well, can be expected with a good optical quality. Otherwise, the images strongly lose details and focus particularly in the border area.
I'll keep it to my macro lens, it is worth it though only slightly, but the optical quality is practically at the same level, and sometimes even a "piece" closer still better ;-)
