When is it the best time to use motion blur (or 2d motion blur) in animation sequences?

it is best to do motion blur when there is a lot of movement in the scene. If someone is walking across the screen then there is not much movement and you don`t need motion blur, if they are running i would only use a small amount of motion blur. It is also better to only do it when the moving objects are nearer the camera. When they are further away from the camera there is less motion blur. I hope this helps...

I think you should either use motion blur all the time or not at all. Motion blur gives the overall animation a different look, and if you have it enabled in some sequences and disabled in others, there is the danger that these sequences don't match from the overall visual appearance. Rendering with a 2D oversampling motion blur like C4D's scene motion blur or ProPack's motion blur will make everything render longer, as it is rendering the whole scene multiple times, no matter if it's moving or not. If you use the 3D motion blur of FireFly or similar implementations (e.g. in PRMan or 3Delight), the render time will depend on the motion - objects that move fast take longer to render, objects that move slower or don't move render only slightly slower than without motion blur.

I have seen movement in film ( On dvds )where there is no motion blur. Watching it frame by frame. I understand what you mean about using it all the time but maybe if you can tell the difference between motion blur and non motion blur then you are overdoing the effect. For something like someone walking there is only a small amount of motion blur. Motion blur shouldn`t make everything move about like the flash. It can be used quite subtlely and i am sure it can be overused. I was only giving SVicious a guide and not hard and fast rules.

Motion blur, when done properly, is not a fancy effect but a simulation of film material. The main motivation for motion blur in Pixar's PRMan was seamless integration in real film footage. CGI without motion blur would not fit into real film, which is always motion blurred. The whole point of motion blur is realism, not being effectful.

I know motion blur is real if you wave your hand in front of your face fast you see motion blur i meant effect in the same way as a nice lighting effect, anyway everything in a 3d program is an effect the pictures are not real.

What i mean is in a 3d program motion blur is an effect as i first referred to it. In reality obviously it is real.

thanks for the response guys reason I asked this was I was curious on when i should use it. a fight scene would be a good use for it right?

A fight scene would be ok