PetKal Offline Upload & Sell: Off
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I have not done that TC comparison although it would be rather straightforward to do so, and I'll make a mental note of it for when I use the lens next.
I'd be really very surprised if the built-in TC doesn't yield better results because it is in effect just another lens element (which happens to be removable) matched to the specific lens.
Although that type of an IQ test has not crossed my mind before, there is another issue which I have wondered about, but never got around checking it out: external 1.4xTCs set the lens AF drive speed back to 75% of its bare lens value. I assume that the built in 1.4xTC does the same thing, however, I'd like to verify that as well. Any time I've shot 200-400L with the internal TC engaged, I have not noticed a drop in AF drive speed, but I have not done a dedicated/direct TC vs. no TC comparison.
In fact, with those new long lenses, it is not easy to observe any AF drive speed drop with external 1.4xTC III, when doing normal "real life" shooting, because AF is so fast to start with that a relatively minor speed drop (25%) is not readily felt. It really takes 2xTC III mounted on a new long lens before the AF drive speed drops quite perceptibly.
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