Hi guys, you may call me a freak, but I spent some time thinking and invented a mathematical formula for calculating the amount of pixel blur you will get. This is the most simplified (usable) version I have been able to come up with... (my first one had like 12 variables :S)
I'm personally not sure if anyone has done a similar math on this, I haven't found it anywhere so yea. =) (can I call it the Ombre's formula for photography blurs? haha) Please do let me know if anyone came up with something similar.
So anyway, just to share with you guys what I've found. C&C welcome. =) Just for disclaimers, I don't study mathematics. (not anymore at least)
B = fxsR / (d - f)
B = Pixels of Blur
x = object speed
s = shutter speed
R = Pixels per metre of sensor
d = distance from subject
f = lens focal length
So how much shutter speed is really needed to "freeze motion"? Say a sports man is running on a field at 4m/s, (this is about running 2.4km in 10 minutes), and you're using a 55-250 IS (handshake blurs are not taken into account). Target is 50m from you.
If you shot at 1/400s shutter speed, the pixels of blur will be 7.82289~ pixels. =) (this is calculation for 350D 8mp sensor).
Perfect accuracy of the formula is not entirely guaranteed, because the distance between your lens and sensor may vary slightly. But this should be negligible with the formula. (at this point i'm not sure).