| Liam Hall |
February 28th, 2010 05:16 AM |
Jon, we are discussing slow-motion here, not frame rate conversions; though related, they are different concepts. I know you understand the concepts but your explanations may confuse many people - if we are simply talking slow-motion, you can shoot 30fps and play back your footage perfectly smoothly in a 24p project, it will just take 25% longer to play than if you played it back at 30fps.
Overcranking, as correctly noted by Mike above, is a film term that has been in use for years. Indeed, when discussing overcranking or undercranking it is useful to think of how it is achieved with a film camera:
Imagine filming the second-hand of a clock; you set the camera to film at 100 fps and shoot for 10 seconds. You will end up with 1000 correctly exposed frames. Now you pop your film print in a projector and set it to run at 25fps, the clock will appear as though it is running 4x slower; it will take 40 seconds to move from frame 1 to frame 1000. Each frame will be perfectly clear and the movement of the second hand will be perfectly smooth.
Next you set your film camera to 25fps and shoot the clock for 10 seconds. Now you have 250 frames. If you play it in your projector at 25fps it will still take 10 seconds to play all 250 frames and the clock will appear normal.
Next, you import your 25fps footage into your NLE and drop it on a 25fps timeline. You add a motion effect so the clip plays at a rate of 25% (there are different ways to do this depending on your NLE), thus your 10 second clip will now take 40 seconds to play. You will notice that whilst the footage looks like slow motion there is blurring on the movement of the the second-hand or it appears to stutter. This is because your NLE has created extra frames by repeating or interpolating them. Of course, there are plug-ins that accomplish this very well, but the best way is to shoot enough frames in the first place.
On a 7D you can shoot at 50 or 60 fps and do a similar trick. But you must change the playback rate of the clip otherwise you are not using all the captured frames and you will simply be repeating and interpolating frames, which wholly defeats the object.
On different systems there are different ways to do this because they use different methods to handle the QuickTime wrapper. On Avid and FCP you'll have to use a program to change the playback rate contained in the wrapper (Cinema tools does this easily). It looks like Vegas has a very neat way to accomplish it as Jon has pointed out above, (though, the only thing I know about Vegas is there are more strip clubs there than in the whole of France).
My advice, test your workflow and check your output frames as Jon has mentioned looking carefully for repeated or blended frames.
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