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A question that I personally wanted to be answered has been answered at The-Digital-Picture with the help of Canon USA.
The answer is actually quite interesting.
What is the Canon RF 100-500mm F4.5-7.1 L IS USM Lens’s maximum aperture opening at 400mm?
Answer 1: When the camera is set to 1/3-stop increments, the maximum 400mm aperture is f/6.3.
Answer 2: When the camera is set to 1/2-stop increments, the maximum 400mm aperture is f/5.6.
Keep in mind that on variable aperture lenses, the camera doesn’t read what the exact aperture is
I would hypothesize that the lens is actually something like f/6 at 400mm and the increment settings just decide how the camera reads the aperture. Rarely are apertures 100% accurate on variable aperture lenses. I haven’t yet seen a patent with this exact optical formula to know what it is at 400mm from a design point of view.