I have seen this question a few times in various forums so I thought it would be another good fit for the “The Basics” section.
Since people rarely explain what they would consider “best”, I assume they are talking about the best shutter speed to achieve a filmic look.
This brings us to the question “what makes a film look like film compared to tv or Canon 5d standard clips” ? Contrary to the general belief, it is not the number of frames per second but the duration of each exposure that creates the look.
In a 24fps clip, each frame is exposed 1/48 of a second and for the next 1/48 of a second, nothing is captured on the film. The 1/48 sec exposure is the key. At this speed moving objects generate just enough blur to make the image pleasing. If you made a 24fps clip using a shutter speed of 1/2000sec, the clip would look much less fluid, you would get the harsh style used in Saving Private Ryan or Crank 2.
The ratio of the shutter duration vs shutter obscuration is often called shutter angle, referring to the angle used to create the opening in the rotating camera disk. Since I am getting technical here, you better look this up on wikipedia, they have a very nice animation to explain it. You can also read Stu talking about the importance of 24fps in The DV Rebel’s Guide.
So, getting back to our original question:
what is the best shutter speed for video on the Canon 5d Mark II ?
If you have been following the explanation, your answer should be 1/50sec (closest setting to 1/48)!
The last step is to convert the 30fps clip to 24fps which is very easy to do for any FCP user as demonstrated in this screen cast from Philip Bloom.