Re: tele extenders

I am considering a tele-extender for my Sigma 7-300 Zoom. At 70 mm, the back element is back into the mount, and moves forward as I zoom to 300 mm. What options do I have and can I preserve the AF? John G.

Lens Converter

I have the Sigma 70-300mm APO Zoom. I would like to put lens converter behind it, probably a 1.4 or 1.7 so I can try to preserve AF. My question is whose do I use? Sigma to match optics(?) or the Pentax or is there another I should consider. John Graves