Canon 1.4 III EXTENDER on 8-15mm fish eye with 12mm EXT Tube

Hi All

Has anyone tried the Canon 1.4 III EXTENDER on 8-15mm fish eye lens with a 12mm EXT Tube on a FF camera? (the extension tube is required due to the Canon extender protrudes quite a way out on the front element and the 8-15mm's rear element is almost flush)

Quite a mouthful but I was hoping it would yield better results than the KENKO 1.4 TC on the same setup. 

I am looking to pick up a Canon 12mm tube and I will give it a go and see what happens.

Concerns will be maximum focus distance but hopefully IQ is improved over the KENKO.

My goal is a portable rig so I can travel with both a CFWA option and a dedicated macro port option for places like Lembeh or Anilao (travelling with a 230mm dome and macro gear seems a bridge too far).  

WAM in lower viz conditions so black or negative space backgrounds will be what I am looking for when possible.

Thanks,

I  doubt it will focus Chris, the effect of extension tubes goes up as focal length goes down.   I don't have a 12mm tube but I tried a 25mm tube between a 1.4x and an 18mm MF wide lens (widest I have available) and the lens at infinity was not focusing even when the front element was touching  the subject.  I also tried a couple of others just holding the lens up to the rear element of a converter the only thing that focused was a 30mm lens at about 150mm from front element.  You can do this with telephoto lenses and still have them focus on nearby objects (but not infinity) but not wide angles.

You can try it yourself just holding the lens in place if you do it carefully, it will still form an image - though the AF lens my not stay at infinity focus when you dismount it - depending on what type of AF system it has.  any light entering through the gap just reduces contrast.

Fair points mate and I greatly appreciate the info.

If I had of looked a bit harder to begin with I could have saved some time.

https://www.dpreview.com/forums/thread/4379406