I've used my Sigma EX 1.4TC on my 200L 2.8 a few times with preatty good results.
Today I thought I'd give it a go on my 135L.
The two images below (100% crops) were taken at F2.8 and F4. The image at F2.8 shows an almost Gaussian blue effect, it's only when you hit F4 does this go.
This isn't apparent on my 200L although this obviously only works from F4.
Is this normal behaviour?