Does Lens Speed Matter?

When shopping for a new lens, you might encounter the desired focal length (or focal-length range, in a zoom lens) in more than one speed. For example, one camera manufacturer’s lineup includes 400mm ƒ/2.8, 400mm ƒ/4 and 400mm ƒ/5.6 supertelephotos. The ƒ/2.8 is 4.5 times larger in volume, 4.2 times heavier and costs $5,000 more than the ƒ/5.6. Is it worth it? Many wildlife and action photographers think it is.