CarlTN said:
My Sigma 120-400 is more than close enough to the sharpness of the Canon 400 f/5.6 prime, especially on my 6D. On a crop sensor the difference is a lot greater. But I have no serious interest in crop sensors anymore. And my Sigma costs less than half of $2000, it zooms, it has IS...and at the wider half of its zoom range it's as good or better than anything Canon makes. The long end really suffers more from a lack of contrast than a lack of resolution, but this is easily corrected in post, or even in camera if you just shoot jpegs. As for the AF speed, it's quite close to the AF speed of my new Canon 70-300L, if not matching it. So your "facts" differ from my facts.
If one needs the sharpest glass for a 70D or a new 1 series with 35+ MP, then I agree with you, $2000 is ok to pay. But for those of us who don't need any more than what 20MP on a full frame resolves, then $2000 for an f/5.6 prime is a waste.
If all you want is the best prime money can buy, but lighter weight and smaller size than the big superteles...in another thread I already suggested that they make something like a 330mm f/3.5 IS with at least one, if not 2 built in TC's. They could sell that for $5k to $6k US, and THAT would be worth it. But a $2000 400mm f/5.6 prime, or a new 100-400 for $3k, is definitely NOT worth the money to me.
I have used several of the Sigma zoom lenses before - my first telephoto zoom was a Sigma (the 80-400) - but when I moved to the Canon telephotos (100-400) there was a world of difference. Although Sigma is very innovative with their lenses - having interesting focal lengths like 120-300/2.8 - 300-800/5.6, and 200-500/2.8 they are not a company to look at for high quality telephotos. While I do give them marked improvement in their shorter focal length lenses, you get what you pay for with their telephotos (and maybe less so with their high end ones).
Personally I only use FF cameras. I own a 5D3 now and started with the original 5D.
Ask anyone who truly cares about image quality and they will all say that TC's are really a last resort option. I own both a 1.4x and a 2x III and only resort to them when I absolutely must. The drop in image quality is simply too great. For that reason I almost never use my 2x - it is almost always my 1.4x on a 70-200/2.8 II. Perhaps some optical engineer will stun us, but right now any lens based off of multiple TCs will be junk.
In terms of justifying $2k for a good 400/5.6 that is easy. I recently justified $11k for an improvement over this lens. Given that I sell large prints of my works, this expense was justified. A few successful images can easily pay for it. I can see amateurs having difficulties with such a price, but anyone who makes money from their work can definitely justify it.