The 72 mm Sigma  close up

Basically, I am a fan of high quality two elements diopters and  find them very versatile tools, especially on a zoom lens. Obviously the quality of a true macro lens is higher, especially at the corners, but with an APS sized  dslr, the crop factor helps to improve the results by cutting off corners. I use extensively the Nikon 5T on my Nikkor 70-210 f 4-5..6 Af on D 70 for outdoor near-macro shots.

This item was sold in bundle by Sigma with some of their zooms (I don't remember which): it is an one element close up lens with 72mm diameter.

It is not an achromat doublet, but having a low dioptric value (perhaps around +0.8) it does not create too many aberrations. Again I bought it  in a second hand market for cheap and used it on the  80-200 f2.8, on the F4 and on the Sigma 135-400 on the D70 (in both cases with a 77-72 step down ring).

Here below are other samples with the lens coupled with the 180 2.8 Af ED: