CyberDyneSystems wrote in post #18435817
I honestly think that the assumption of better overall AF in the f/2.8 is also flawed.
You should check Canon's technical information about active points and types vs max aperture.
Some AF points and types can only be used with f/2.8 or larger apertures.
This is not an assumption, but a fact.
The new 100-400mm has better AF than any of my 70-200mm f/2.8s had (mind you I don't have the latest)
Obviously, apart from aperture, the AF motor and lens firmware will greatly affect AF performance.
You cannot compare new lens models to old ones and use that to prove your point.
If all else is equal yes, f/2.8 will help in low light,. but looking at modern Canon AF systems the need for f/2.8 precision is becoming more and more marginalized.
It is becoming less of a limitation for the few people who can afford the latest and greatest cameras.
Lastly, the chosen example was a poor example IMHO for number of reasons, not the least of which the aging 1990s design 400mm f/5.6L which costs 1/12th what a modern f/2.8 costs has lightning fast Af that comes very close to being as good as the nearly 20 years newer model. That f/5.6 absolutely can track a BIF BETTER than it's contemporary f/2.8 counterpart (the pre-IS version)
Uhm... I would be quite surprised if the former could track BIF better than the latter in all but cherry-picked test conditions. On what do you base your "absolutely better" ?
Also, keep in mind that the 400/5.6 is still in production, and in over 20 years Canon has probably updated AF motor and firmware in the lens a few times without advertising it. Recent copies might be better than older ones, also depending on which camera is used.