Worthwhile: Canon TC1.4x III used with older 500 mm f/4L IS and the new 100-400 II

[Dennis Zaebst]

I have two 1D-IVs, one 1D-X, a 500 mm f/4L IS (older 500) and the new 100-400 L IS II, plus the 70-200 f/4L IS. I also have the older 1.4x II and 2x II teleconverters. I am wondering if it is worthwhile purchasing either or both of the new 1.4x III and 2x III teleconverters.

Will there be any gain in optical sharpness or AF with the 500 or the 100-400 II moving from the 1.4x II or the 2x II? I see reviews online saying both of the III TCs are sharper than the II TCs (on BHPhotovideo.com and Amazon.com). Is this true, especially given my main telephoto lenses?

In short, is the purchase of either of these newer TCs worthwile? I probably won't be investing soon in the new 500 or 600 Mark II lenses due to finances as they are.

[arash_hazeghi]

If you search the forum there are many threads about this topic:

1.4X III : Most of the advantage is in AF but that's only with the new lenses. Sharpness is about the same as 1.4X II.

2X III: It is sharper and has less CA than 2X II on any lens. Again AF is only better with new lenses.