100-400 in spring?

ijm5012 wrote:

Yep, this is the lens I'm holding out for. It will be a fantastic lens for wildlife or motorsports. I will also be interested to see if the rumor of it being f/4 from 100-300, and then going to f/6.3 from 300-400 holds true or not.

It can't be true. It is possible to calculate the maximum possible aperture at every focal length, assuming you know the max entrance pupil of the lens. And the max entrance pupil usually comes at the max focal length. So, 400/6.3 = 63.5mm.

Assuming, the entrance pupil never gets bigger than 63.5mm at any focal length, then the MAX possible at 300mm is F/4.7. It will probably be slower than that though.

Sorry.