Shutter speeds for stopping motion at different focal lengths

[Feanor]

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?

[HiFiGuy1]

I may be way off, but it does make sense to me. If you are shooting with a 200mm and a 400mm, for example, and you shoot the same subject at the same physical distance, the framing is very different, and therefore the same subject motion takes up substantially less linear distance (half?, not sure) at the same shutter speed (Time value) within the confines of the plane of focus. Put another way, the angular velocity would seem to be proportionately less with the shorter focal length. Therefore, to stop the motion equally in each shot, the 200mm would need only half the Tv versus the 400mm, if my spatial assumption is correct.


I'm sure Daniel can jump in and shoot down my off-the-cuff hypothesis, in an eloquent and technically astute way of course, and I look forward to the lesson.

[neuroanatomist]

HiFiGuy1 is correct. For a subject moving at the same speed, a longer lens requires a faster shutter speed due to the tighter framing. Of course, that's a generalization since subject speed and angle of motion relative to you are also significant factors.

[Feanor]

I may be way off, but it does make sense to me. If you are shooting with a 200mm and a 400mm, for example, and you shoot the same subject at the same physical distance, the framing is very different, and therefore the same subject motion takes up substantially less linear distance (half?, not sure) at the same shutter speed (Time value) within the confines of the plane of focus. Put another way, the angular velocity would seem to be proportionately less with the shorter focal length. Therefore, to stop the motion equally in each shot, the 200mm would need only half the Tv versus the 400mm, if my spatial assumption is correct.


I'm sure Daniel can jump in and shoot down my off-the-cuff hypothesis, in an eloquent and technically astute way of course, and I look forward to the lesson.
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I understand what you're saying and it's why in my OP I talked about proportion of viewfinder filled, rather than physical distance.


But surely physical distance is nothing to do with focal length? If I take a photo of a bird at 200mm and crop it so that it has the same print size as the same bird shot at 400mm the motion blur will again surely be the same. It's only less noticeable on the uncropped 200mm image because the bird is smaller and therefore any blur is smaller. Or am I missing somethng?

[neuroanatomist]

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

It's physical distance relative to angle of view for the lens. With the 24mm's wider field of view the object would take longer to traverse the frame (even if positioned so the object was the same relative size as with the 400mm), meaning at 24mm the object would be relatively slower, requiring a slower shutter speed to stop it's motion.