background action in fast motion

[oliver imfeld]

i need to film a dancer in front of a big crowd (crowd in a public, busy street), the dander will move at 25% of his normal speed, background is moving at 100%; in post we intend to re-accelerate the dancer's movements up to 100% in order to have him at "normal" speed and therefore all the background is moving very fast; the option of doing this on green screen has been aborted for several practical reasons; therefore i would like to know if there are particular points i need to consider or if i just should go with my most usual settings:25 fps @ 1/50, fstop +- 5.6 ?
thx for any advide, cheers
oliver

 

[Cail Young]

You may want to experiment with shutter speeds, other than that sounds right.

 

[Brandon Fraley]

if you are having the dancer "mime" slow motion in front of the crowd and then planning on speeding it up, just shoot it at the frame rate you want rather than 25 and reinterpreting the frame rate in post. Like Cail started to imply, you can get weird shutter artifacts if you change the frame rate in post (which you may want, I don't know), but if you want a natural 24 frame look, shoot with the desired frame rate and 180deg shutter from the get go.

 

[Adam Hebert]

Consider shooting tests at 6fps, with the dancer at 25% and BG at 100%, since you are gaining 2 stops of speed (4x) it should produce the desired effect, hopefully the shot is static or else you will run into problems. Also, plan to ND6 to get your desired f/stop if you really want a particular depth of field.
I'm pretty sure thats exactly what Brandon said, but whatever.

 

[Chris Newman]

I agree with 6 fps 1/12 sec (180 deg) shutter. If you have control of the background extras, make sure they never stay still for too long, so they will always be obscured by motion blur, which will keep them from attracting the viewers' attention. You might even test 3 fps and doubling frames to increase this effect. I would also have the dancer wear color(s) that contrast with what the extras are wearing.

 

[Adam Hebert]

I agree with 6 fps 1/12 sec (180 deg) shutter. If you have control of the background extras, make sure they never stay still for too long, so they will always be obscured by motion blur, which will keep them from attracting the viewers' attention. You might even test 3 fps and doubling frames to increase this effect. I would also have the dancer wear color(s) that contrast with what the extras are wearing.

Chris,

Could you explain the effect achieved by doubling the 3fps?

 

[Chris Newman]

Chris,

Could you explain the effect achieved by doubling the 3fps?

The concept is kind of like these:
http://ecx.images-amazon.com/images/I/51HX63XZ40L.jpg
http://images.inmagine.com/img/photoalto/paa014/paa014000007.jpg

The subject stands out because the background people have motion blur. I haven't actually tried it in a film, but it could be worth a try.

 

[Adam Hebert]

So doubling the 3fp would increase the motion blur relative to the 6fps shot?

 

[Chris Newman]

Yes, because the shutter speed would now be 1/6 sec vs. 1/12 sec. I have seen this effect used in films. I can't think of examples right now, but I think they are usually for POV-type shots when the person is tired or drugged. In this example, with the camera locked down (or on a dolly), I think it would evoke a different feeling.

In any case, also beware that the dancer's motion will probably be a little awkward and not very smooth from being sped up.