JML 21 mm, full picture reduced
JML 26 mm, full picture reduced
JML 21 mm, detail 1:1 (near the top left corner)
JML 26 mm, detail 1:1 (near the top left corner)
This is not an exhaustive test, but it seems to show that the performance of the two lenses is rather similar. There is a difference in theoretical max resolution in favor of the 21 mm (at the same magnification) because of the lower f/ratio, but this difference is not substantial. In fact, my calculations show that both lenses can be used to approx. 5x before becoming diffraction-limited in a visible way.
The subject is not plane and not in focus in the whole frame, and there are other shortcomings as well. Nonetheless, the results look promising.
And last, a non-stacked image with the 26 mm (the subject is scales on a curculionid beetle), reduced and 1:1 detail
The odd thing about the last test is that the curculionid pictures were taken with a Mitutoyo FS-60 microscope equipped with zoom infinity lens (set to 1x for this picture), so the test is not fair - but the JML 26 mm still produces rather acceptable results, at least in the center of the frame. Magnification is also higher than optimal (around 8-10x).
