Well, if you're taking pictures at 240mm, why don't you compare your pictures with a 70-200 f2.8 zoom instead, as that's a much closer focal range? I get it that f7.1 is slower than you want, but it's only 7.1 because it's at a 500mm reach. If you want faster than f7.1 at 500mm with non-cropped FF then you'll have to pay big bucks for a huge & heavy lens (well, unless they get DO optics of sufficient quality involved).Saw a couple mallard ducks out back and some deer. Getting close to twilight last night, but not dark by any stretch. I have an RP and an RF 24-240. At 240, I had to be at 6.3 and 1/100th for decent exposure. Pushed the ISO right to 6,400 for the brightness I wanted (just a couple JPEG snaps).
Long way of saying that 7.1 during the so-called “golden hour” could be problematic for people on the 100-500. I’m happy with my pictures, but wouldn’t be if I had paid a couple grand for the lens by itself.
Maybe Canon/Sigma should start thinking about having a set of 75mm aperture tele zooms which would be something like a 100-300mm f2.8-4, 135-420 f4-5.6, 200-600 f5.6-8. Then you could get a 100-300 f2.8-4 to compare with your 240mm f6.3, or a 135-420 f4-5.6 for a f5.6 worst case.