I have 14mm 2.5, 20mm 1.7, and Ai-s 50mm 1.4. Definitely not interested in Sigma 30mm 2.8.

@shian used 0.7x converter on his Ai-s 50mm 1.4 http://www.personal-view.com/talks/discussion/comment/61642#Comment_61642 Smart idea. Less # of lenses. Easier color grading.

I'm looking for 0.6x converter to turn Ai-s 50mm 1.4 into 30mm 1.7 if one third stop light loss.

Calvision has 3 different flavors for 0.6x converter. No idea which one will work if it works. http://www.cavision.com/optics/industrial/adapters.htm

Any advice is greatly appreciated.