Upload & Sell: Off
I couldn't resist trying this combination. I have a 24-105, a Kenko Pro DG 300 1.4X, and a 5D II.
24-105 @ 70mm + 1.4X, f/5.6 effective, 100mm effective:
24-105 @ 70mm + 1.4X, f/11 effective, 100mm effective:
24-105 @ 105mm, f/4:
24-105 @ 105mm, f/5.6:
AF performance was the same in daylight with or without the TC. It was slower indoors with the TC. The comparison shots were all manually focused.
Wide open, center performance hold ups with the TC. Sharpness falls off rapidly at the edges. Upsizing a file from the bare lens at f/5.6 would give better results for most of the frame. Stopped down to f/8 (f/11 effective), the TC gives a much stronger performance that you won't be able to match upsizing.
The TC holds up at all apertures, though I agree with the suggestion earlier for an 18-200 or 18-135.