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Shooting for slow and fast motion: increase shutter angle speed or FPS, or both...? Pro and Contras


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#1 Duca Simon Luchini

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Posted 17 June 2017 - 03:07 AM

Hallo everybody,

more and more videos that we can see around, have a slow motion and fast motion applied (i thing i post). So we can see for example a drone above the city flying quickly, and slow down quickly to almost stop, then maybe resume flying quickly or at normal speed ... (three type of velocity in the same shoot!).

This is something we can make entirely in post but my question is: how can we properly shoot this sequences on set, to avoid motion blur and to have enough FPS to make e believable extreme slow motion... (and for many other VFX needs, like Keynig, match-moving...)?

 

Shortly, we should increase the shutter angle speed (and break the 180° shutter speed rule...) or augment as much as possible FPS (Depending on the camera's capabilities, of course)?

Or maybe both, but in which proportion?

 

i.e.

I mean, the first idea could be to increase FPS to 50 or 60 fps (common DSRL in use). According with this frame rate, I should increase shutter speed to 100 or 120 to stay in the 180° shutter speed rule, if I'm right.

The problem is when you increase extremely the shutter speed, to 1/1000 and many more... in these cases you cant increase your frame rate... so what happen?

Of course, if you increase shutter speed, you underexpose and you have to compensate with Aperture or adding lighting. But without exposure problem, again, what happen if you increase shutter speed breaking 180° rule?

 

P.S. My question is concerning DIGITAL SHOOTING, no FILM.

 

Many thanks for a reply.

 

 


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#2 David Mullen ASC

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Posted 17 June 2017 - 09:21 AM

Not sure I understand your question -- if you want smooth slow-motion footage, then shoot at a higher frame rate.

 

Frame rate affects shutter time, so at 60 fps, for example, you cannot physically have an exposure time any longer than 1/60th of a second because the camera is taking a new picture every 1/60th of a second.  Normally you'd use a 1/120th of a second shutter time at 60 fps (i.e. 180 degree shutter) for traditional motion blur rendition, you can use a shorter time if you want less blur but more motion strobing (choppiness, crispness) or a longer time if you want more blur.  

 

At super high frame rates, higher than a DSLR can shoot, there is less of an issue with strobing or blurring because you have so many motion samples.

 

There are also ways in post today of smoothing out choppy motion (but the opposite is harder, it's hard to make blurry motion more crisp.)

 

When you say "increase shutter speed", what do you mean?  Increasing the time so that there is more motion blurring?  Or shortening the time so there is less blurring?  Which direction from 180 degrees are you wanting to go?


Edited by David Mullen ASC, 17 June 2017 - 10:10 PM.

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#3 Duca Simon Luchini

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Posted 17 June 2017 - 09:38 AM

Hi David,

First I ask if shutter speed must be twice the frame rate, because this I read...

If shutter speed should be twice the frame rate value, then you should use 1/120th  and not 1/30th... sorry, I am confused... :wacko:

 

And of course, I don't wont blur...

 

I mean. if you shot at 24 fps, (film Shutter angle 180 degree) you should set up shutter speed to 1/48th to have a "natural blur";

If increase frame rate, to 60fps, e.i., you should al least set up shutter speed to 1/120th, and you have still more frame per second (good form slow motion)  but still remains a "natural blur". To remove much mor as possible this blur, what have you to do? For example, if you shoot at 60 fps and increase shutter speed to 1/1000 you should be remove more of the blur... of course you also underexpose, but you can add light ot change Aperture...


Edited by Duca Simon Luchini, 17 June 2017 - 09:49 AM.

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#4 David Mullen ASC

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Posted 17 June 2017 - 09:58 AM

Yes, you can use a shorter shutter time to reduce blur per frame.

 

There's nothing wrong with doing this, I've seen some commercials that use high frame rates with a closed-down shutter angle on a film camera (less than 180 degrees, like a 45 degree angle) to make the motion very crisp.  Yes, you need a lot of light. I saw a cool commercial of runners on a dirt track shot at very high frame rates with very short shutter times to make very drop of sweat falling and every bit of dirt flying up very crisp because of the reduction in blur.

 

The main problem is just if you want to do a speed change in post back down to 24 fps, where you'd normally want the look of a standard 180 degree shutter.

 

For example, let's say you shot at 72 fps with a 180 degree shutter - so the motion blur per frame is that from a 1/144th shutter time.  So when you changed speed in post to 24 fps, your per-frame exposure time is still 1/144th, which is very short and you end up with motion that looks very choppy as if you shot it with a very narrow shutter angle.

 

So while there is nothing wrong with shooting slow-motion at 60 fps with a 1/1000th shutter time instead of 1/120th shutter time, to reduce motion blur and get crisper motion, it makes the problem of going back to 24 fps and having normal motion blur even harder where you'd expect the blur to be that of a 1/48th shutter time.

 

However, as I said, today there is motion interpolation software that can blend multiple frames to make motion look smoother (i.e. blurrier) so you can change from 60 fps to 24 fps and get back to something normal-looking.

 

It's just a hard fact of life that 24 fps needs a certain amount of motion blur per frame to not look strobey and choppy because it is inherently a rather low frame rate to use to achieve smooth motion.


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#5 Robin R Probyn

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Posted 17 June 2017 - 08:36 PM

"Normally you'd use a 1/30th of a second shutter time at 60 fps (i.e. 180 degree shutter) for traditional motion blur rendition,"

 

David I think the op was confused by the above in your previous post.. I believe it was a mis type .. and should have been 1/120th..?

 

​Duca.. I shoot quite a bit of moderate slo mo for corp videos .. as you note its very trendy these day .. :)..  I always go for the double frame rate /shutter speed  180 degree rule.. or very close to it.. if light allows it.. for around 100 to 240 fps range..

 

​Recently I shot 100 fps at 1/100th..(low light location) i.e. 360 degree shutter.. it was ok as the subject wasn't moving that fast..but it was a bit too blurry to my liking .. 1/250th I found to look better... so yes I would go with the 180 rule.. 

The main thing to worry about when shooting interiors is to avoid flicker from practical lights.. esp fluorescence lights.. in say an office ,even way in the back ground its very noticeable  and can ruin a shot.. you have to match the frame rate with the mains supply rating .. e.g. shoot 100 fps in 50 Hz.. 120 fps in 60 Hz countries ..   

 

 

That ramping slow to normal thing is all done in post.. 


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#6 David Mullen ASC

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Posted 17 June 2017 - 10:09 PM

Yes, that was a typo, I meant 1/120th for 60 fps.  I'll fix that...


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#7 Duca Simon Luchini

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Posted 18 June 2017 - 05:38 AM

Okay guys, very well and many thanks for your exhaustive replies (as always!): 


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#8 Robin R Probyn

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Posted 18 June 2017 - 06:24 AM

The ramping..as I know it.. (Im not an editor or post prod)..   you shoot the whole shot high speed..then in edit you speed up the slow mo to normal speed.. then have an in and out key frame for the part of the shot you want to go back to slo mo ..  this is the problem with a very high shutter 1/1000th @ say 100 fps.. when you speed up the shot (i.e. make it normal 25p/23.98p) it will look strange . strobe.. stutter.. not normal motion blur.. as David explained .. 


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