While browsing some forums I've come across comments about needing faster shutter speeds at longer focal lengths to stop motion. This doesn't make much sense to me.


For example, I've just got the 400 f5.6L prime that I ordered for myself for Christmas. I understand that when shooting handheld I need to use at least 1/400th, ideally 1/500th or 1/640th. But one post I read on another forum said you need a minimum of 1/800th to stop motion with this lens.


Surely the focal length makes no difference to stopping motion? If you shoot an object at 400mm and then shoot the same thing at 24mm, and the object fills the same portion of the viewfinder and is moving in the same way at the same speed in both shots, surely the shutter speed needed to stop that motion is exactly the same?