Blurry Canon EF 100mm f/2.8 IS
One reader suggested that the blur is caused by the IS (image stabilization) mechanism being turned off (as is advisable for tripod use). I saw no improvement with it enabled.
I compared the third (bad) sample of the Canon EF 100mm f/2.8L IS, I to the Zeiss ZE 100mm f/2 Makro-Planar, one of the Zeiss ZF.2/ZF/ZE line which I review in depth in Guide to Zeiss ZE / Zeiss ZF Lenses. The 100/2 Makro-Planar is my reference lens.
These crops are from the left edge of the frame, the full review has left/center right and more apertures.
The Canon has a 3.1% linear advantage, but still looks like scat-like. I have no supposition that this is a design issue, rather it must needs be a badly assembled lens, the third of three bad samples.
Stopping down to f/5.6 helps of course, but the Zeiss at f/2.8 is vastly superior to the Canon at f/2.8. Actually, the Zeiss wide open at f/2 is superior to the Canon at any aperture!