muteteh said:As far as my limited knowledge in lens optics goes, 400mm / 5.6 = 71.4mm, so it should be possible to make this lens with 77mm thread size. If I should guess, such a lens would be too expensive for me to buy.
As for an EF 100-300mm f/4, my point is it would directly compete with other Canon lenses. If price was not an issue, why would one prefer an EF 70-200mm f/4 over the EF 100-300mm f/4 ? Same on the longer side - why prefer an EF 100-400mm f/4.5-5.6, if you can buy an EF 100-300mm f/4 + EF II 1.4x ?
The size, weight and price should be similar to the 70-200 f/2.8 IS. The same goes for the filter size. By adding a 2x TC, you already get this range of focal lengths and apertures, so an optimized design should be more compact and lighter and, hopefully with better IQ that the 2x TC combination.
The only lens in direct competition is the old 100-400 f/5.6. A 200-400 f/5.6 would be a replacement for that lens. The price would be much higher than the 70-200 f/4, so no direct competition there.
Why buy a 100-300 f/4 plus 1.4x TC instead of a 200-400 f/5.6: Hopefully better IQ. Following that argument: Why would anybody buy a 100-300 f/4 instead of a 70-200 f/2.8? Why would anybody buy a 600 f/4 prime instead of a 400 f/2.8 plus 1.4x TC?...