Canon Extender EF 1.4x III Review -

RS_RS wrote:

The 2× Extenders have always been a bit marginal, apparently working best with the Big White Lenses, and tests suggest that there has indeed been an IQ improvement from each version to the next. Your incentive for buying the 2× III is that it takes f/8 AF coverage above the default with the 200~400/4. What? You don't have one of those yet?

I'd like one... even if it's just there for occasional use (2x) but I'd actually like a 200-400/f4L - it's likely to be the next lens I buy. I think your observation on the Extenders working best with the larger White lenses is probably spot-on. Those lenses tend to produce excellent results to begin with. I'm using the 100-400mmL II as a substitute for a (better) telescope at the moment and I'd eventually like to strap it to a tracked mount so I can do longer exposures of celestial subjects.
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But the reason I'd like the EF 2x III lens is more for lunar shots. I like the results from a cropped sensor camera but I'd like to use a FF camera with a little more magnification that the 2x might offer. I've only heard of three people speaking kindly of the 2x III extenders, hence my hesitation. Nearly everyone I knew who used a Mk II was a little disappointed with the results and that's the reason I changed my mind at the last moment and bought the 1.4x III instead. Now I'm becoming swayed back towards the 2x for the added magnification. I guess if you need it, you just have to get it (eventually)! I have a Canon ET EF 12 Extension Tube that I can use to mount the two together but it's quite a shame Canon ended the ability to stack these. I guess using two is going to reduce image quality further. Either way, I'm happy with the image quality from the EF 1.4x III.