…just cracking up that Canon for weight reasoning did not make the TC 1.4 part of the lens ... I guess convenience was not part of the development plan, yet they added it back into the 200-500 f4 which they claim is lighter than the 500. I don't see the upside to anyone, you, me, Canon, to have left the TC out. I'm also concerned about sharpness with an on board TC vs one you added on, I would figure being part of the lens would yield better results.
Sorry, but your post is a bit cracked.
First off, about the 100-300/2.8 Canon stated, “
We also considered a lens with a built-in extender, but we decided to achieve 3x zoom without a built-in extender as it offered the best balance between size, performance, and spec.” It was about more than weight. I’d guess it was mostly about length, the lens is already significantly longer (75 cm / 3”), a built-in TC would have added significantly to that difference.
Second, they haven't 'added it back to the 200-500/4' nor have they 'claimed it's lighter than the 500/4'. Canon has said nothing about a 200-500/4. You seem to be confusing rumors with reality. Yes, Canon patented a 200-500/4 + 1.4x. They also patented a 100-300/2.8 + 1.4x, but they made the lens without the TC. As for the upside of leaving out the TC, it's obvious that Canon saw one, since they did just that even though they obviously considered (and patented) a design with it.
The claim of 'lighter than the 500/4' is something stated by CRguy. Certainly just making the prime into a zoom is likely to increase the weight, as we saw with the 300/2.8 II (which is the same design age as the 500/4 II). Adding a TC would only increase that difference. Not sure why CRguy claims that, but he's arguing with physics and that's never a good idea.
Regarding the TCs,
@john1970 previously posted the MTFs of the 100-300 w/ TCs, along with the 100-500. I suspect you're right that an internal TC would be optimized and result in less of an IQ detriment than the external version. However, the resulting 140-420/4 bests the 100-500, and the 200-600/5.6 is not much worse. As the MTFs above show, the bare 100-300/2.8 is as sharp as the EF 300/2.8 II prime, which is truly impressive given that the prime is one of the sharpest lenses made.