Originally posted by leonine
3. Why is the 2X converter rated inferior to the 1.4X converter? Why is it so much cheaper? I have used it in combination with both the 300mm and the 400mm lenses. I wasn't happy with the result despite taking test shots on a sturdy Tripod and head in broad daylight.
All excellent questions but I can only answer one of them. From my experience, regardless of brand and format, all 1.4x teleconverters have better IQ than their 2x versions. Part of the advantage is that with 1.4x you only lose one EV or stop, whereas with a 2x you lose two EVs or stops. That loss of light results in either the need for higher ISO, slow shutter speeds, and/or less DOF.
Also the 2x is essentially cropping more of the image circle and the resolution limits of the lens are more apparent than a 1.4x teleconverter. The price/cost of the two has little to do with the cost of manufacturing and everything to do with demand and value.
It would be really interesting to do a test comparing a (a) 600mm telephoto vs. a 300mm with a (b) 2x teleconverter vs. (c) 300mm telephoto cropped to the equivalent of the 600mm field of view. I know "a" would have the best IQ, but would be curious to see the difference between "b" and "c".