Zoom lenses with a lower f# at their wide angle end always (IMHO) seem to quickly have their f# approach the f# of the telephoto end as they go through their range to telephoto. Therefore I'd bet that the f# of the 100-500 f4.5-7.1 at 400mm is in the far upper 6's or even 7.1.
Even with that said, I see a very strong value in this lens coming out. It will a be more affordable, shorter & lighter weight lens for those who can just afford it and want to handhold it (probably people like me).
In the future, I look forward to Canon coming out with yet another long telephoto zoom that is more like a 135-500 f4(or f4.5 or f5) for those with deep pockets and a willingness to buy such a beautiful, yet big & heavy lens. - Why 135 at the wide end? I'm just guessing that this could be part of the wide f2 trinity of zooms.
1) It's relatively interesting discussing where the RF 100-500mm will change apertures through its zoom range. For example:
- Canon EF 100-400mm f/4.5-5.6L IS II USM changes from f/5 to f/5.6 at 312mm.
- Canon EF 70-300mm f/4-5.6L IS USM changes from f/5 to f/5.6 at 229mm
- Sigma 150-500mm f/5-6.3 DG OS HSM Lens changes from f/5.6 to f/6.3 at 313mm
Hence with this data and the design aspects of this lens we know, I guess the RF 100-500mm will change from f/6.3 to f/7.1 at ~320-400mm.
2) What's much more important to me, is how well the R5 (and other/future Canon mirrorless cameras) will AF at various light levels. My experience bears out that AF ability of focusing on subjects is not always entirely linear (on light level, max available aperture). Some of my DSLRs with live view and/or DPAF, as well as my M5 and my use of the EOS R have indicated differing AF characteristics (e.g. also more dependent on contrast / colour, type of lens, etc).
I know that the R5 is not primarily designed to be a 'sports or wildlife camera'... However, I still do 'wish' that the RF 100-500mm would focus "fairly well" (consistently, reasonably fast, and without too much hunting) on the R5 in relatively low light. I would love to use the R5 for some twilight birding, event photography where lighting can often be tricky, as well as the occasional sports. Ideally, my hope for decent AF in this manner, would apply to all of its AF modes (one shot, tracking, face/eye/animal, etc).
Then in additional, I also would like the R5 will focus very well at extremely low EV light levels with brighter lenses (f/2.8 and below), and especially so with primes f/2 and below. That's where it's at for me. Hopefully we'll know the answer to that when thorough reviews and early adopter experiences come through.
PJ