Effect of a teleconverter depends on the lens?

So, I watch photography videos on Youtube from time to time. Now I came across this one: http://www.youtube.com …pvQZT22k&featur​e=g-subs-u by Gregory Cazillo, probably known by some of you.

He says (look at the comments as well) and I quote:
'Each lens is affected differently by TCs, some are only 2/3 of a stop with a 1.4 others are 1 or 1 1/3 stops. All depends on the lens/TC/zoom position combo.'

That's the first time I hear that and I have a hard time believing that, based on my personal experience with my Kenko 1.4 Pro 300 DG and several lenses. In my opinion, a 1.4 teleconverter ALWAYS makes you lose one stop of light regardless of what lens you use or what zoom position.

What are your opinions on this/what is the reality?

I think you are right and Gregory Cazillo is wrong. It is simple physics. All lenses will be effected the same way. Maybe he is confused because some teleconverters report an accurate f/stop, while others don't.

The FL is multiplied by the magnification of the teleconvertor, while the opening size of the aperture (in millimeters) is unaffected.

So if you mount a 1.4x TC on 100mm f/2 lens (50mm aperture diam.), you get 140mm FL with a 50mm aperture or 140mm f/2.8 end result. Simple arithmetic about FL change and the light gathering capability of the resulting combination.

Cazillo statement is incorrect as stated originally; perhaps there was some different issue that he was raising. For example, put the same 2x TC on two different 100mm lenses, the first one which has high IQ and the other which has mediocro IQ, and the resulting combination might result in unpercievable quality loss with the first lens but terrible results with the second lens because IQ imperfections are 'magnified' by the TC

As the TV ad parodizes, not everything posted by folks on the internet is accurate. And as Mythbusters has proven, some of the things posted on YouTube by folks are not what they seem.

msowsun wrote in post #15230193
Wouldn't a 100mm f/2 with a 1.4x TC become a 140mm f/2.8? (140mm focal length divided by 50mm aperture = 2.8)

Yes, you're right...brain f*rt, I must have taken two 'stupid' pills by accident this morning, making me too stupid (going back to fix the earlier post)

If you read what he wrote in the comments and what he says in the video (at one point he says that putting a 2x tc on a f/4 lens will give you f/5.6 depending on the lens...), I don't see how you could see this in another way.

I guess he hasn't used teleconverters before and just wanted to help a beginner who had a question but didn't possess the full knowledge himself.

I was trying to give the guy the benefit of a doubt, rather than make him appear to be totally clueless.