close-up lens' versus extension tubes

Olaf Ulrich wrote:

http://www.ulrix-photo.de/misc/closeup/en/closeup.html

This web page compares the results shooting a flat subject with three 50 mm lenses on an APS-C-format DSLR camera at 1:3 magnification:

1. Dedicated 50 mm macro lens

2. 50/1.4 lens with +5.8 dpt close-up diopter lens

3. 50/1.4 lens on a 17 mm extension tube

The combination #3 yields good center sharpness and definitely is good enough for most typical three-dimensional macro subjects, but clearly falls off in the corner for a flat subject. In contrast, combinations #1 and #2 are almost equally good, with perfect sharpness from center to corner.