Should I purchase a 300mm f/2.8 is I for wildlife???

[ethanz]

But also, don't f4 lenses focus faster than f5.6?

 

[takesome1]

But also, don't f4 lenses focus faster than f5.6?

Not necessarily. It depends on the lens.
With the 500mm F/4 the lens may be moving more mass inside.
Battery power, (other bodies vs the 1D line) seems to matter more for speed on the bigger lenses.

One stop is an 8 to 10 minute advantage in the morning and evening. It may not sound like much but you gain time at the most prime wildlife time.

 

[Graphic.Artifacts]

F4, 2.8 and 2.0 aren't just about shutter speed and ISO. The fast telephoto's draw like portrait lenses. Even the 400 5.6L which probably isn't quite as tack sharp as some of the newer L glass renders very nicely IMO.

 

[ethanz]

Not necessarily. It depends on the lens.
With the 500mm F/4 the lens may be moving more mass inside.
Battery power, (other bodies vs the 1D line) seems to matter more for speed on the bigger lenses.

One stop is an 8 to 10 minute advantage in the morning and evening. It may not sound like much but you gain time at the most prime wildlife time.

I meant not in terms of the lens but the AF sensor in the body.