Quote:
Originally Posted by
dmiller68 
My brother has the Sigma x2 converter that he is going to sell me so then I can extend it further.
I do not think you will like the results of adding a 2x TC to a zoom lens that is already f/5.6. The TC will give you a maximum aperture of f/11, no auto focus capability, and probably very poor image quality. You would be much better off cropping the photo in post processing.
A 2x TC is really only useful on a f/2.8 lens, and then you better have a very high quality prime lens and TC combo. I've seen photos with a Canon 70-200mm f/2.8 L IS MKII with the new Canon 2x TC III that are still not as sharp as the Canon 100-400L, and that is the highest quality zoom lens + TC combo you can find.
I have a 1.4x TC that I used to use with my 70-200mm f/4 L IS, making it effectively a 98-280mm f/5.6 lens. It worked well, but I rarely use it now that I have the 100-400L. Adding the 1.4x TC to the 100-400L does not give very good results, and I lose auto focus on the 7D body. I get better results cropping in post processing.