cdifoto wrote in post #18767295
Sigma 150-600 is 6.3 on the long end. And probably pretty early on. (I'm not looking it up). That's not really a miracle. 6.3 is slow as molasses.
For what purpose? Compared to what? "f/6.3" has no absolute practical meaning, except perhaps, to PDAF, which sees f-ratios as an absolute factor. For capture, f/6.3 only tells you about exposure from a grey wall as a subject, with a given shutter speed. 600/6.3 gets 2/3 stop more light from a subject, from the same distance and with the same shutter speed, as a 300/4, no matter whether you put a TC on the 300/4 or not. Except for the PDAF issue, it's about proximity and entrance pupil size. Have someone point a 24/1.4 at you and someone next to them, a 600/6.3. How do the light-catching entrance pupils compare? The 6.3 gets 31x as much light, almost 5 stops more, from your face, per millisecond. You normally don't notice how much noisier the 24/1.4 would be, because you don't magnify the face that much, because it falls apart for other reasons like pixelation.
It should be a given that the more magnification you have, the higher the expected typical f-numbers will be. it is the entrance pupil that determines the ratio. 600/6.3 only captures 1/3 stop less light than 400/4 with a 1.4x TC.
I had a Bigma. Trust me it ain't that useful.
There is no way in hell Canon can make an F/4 smaller than Sigma's 150-600mm. The camera body isn't gonna make a difference. It still needs to resolve to a full frame sensor.
What does it mean to "resolve a sensor"? That's a very strange way of looking at things; sensors are what resolve the analog projections of the lenses. Anyway, the diameter of the front optical element of the lens must be at least the maximum focal length divided its minimum f-number at that focal length, plus the sleeve to hold the front element.