Shutter speed; how exactly does it work?

[Crashsune]

There's an extremely low amount of information about this setting online, and I'd really like to know more about it.

Firstly: Does adding more shutter speed make more motion blur, or less?

Secondly, when deciding what amount to use, I read somewhere that for 24 fps, (possibly 48 as well) you're supposed to use increments of 0.021. Is that true? If so, why?

Thanks

[Marco Skoll]

It sort of doesn't (work, that is) - it's often one of the buggier parts of SFM.

1) That kind of depends on how you define "more speed". The setting is exposure time, and a longer exposure time means more motion blur, but you could rather argue that a longer exposure is actually less speed!

2) Not heard that one before, and it doesn't actually make sense, as a 0.042 setting (your first multiple) would actually mean frames were overlapping (as there's only 0.041666... seconds between frames at 24 fps), and that could potentially really confuse SFM when it's trying to render.

Certainly, it's not true to reality.

The main limitation seems to be that SFM disables it entirely if the exposure time is too low (or possibly when samples are too close together).

[Zappy]

The shutter speed is the amount of seconds before and after a frame to sample motion blur through. For example, a shutter speed of 0.5 will sample motion blur from half a second before to half a second after the given frame.

Therefore, the recommended method of choosing a shutter speed amount to use is to divide 1 by the amount of frames per second in the session, and then use half of that as the shutter speed amount.


For example, with 10 frames per second, you divide 1 by 10, and get 0.1. Then you halve that and use 0.05 as a shutter speed amount. (If you just used 0.1, the motion blur would be sampled from 0.1 seconds before to 0.1 seconds after a given frame... and as the previous frame was just 0.1 seconds before, and then used 0.1 seconds afterwards for motion blur, that'd leave a 0.1 second (1-frame) overlap of motion blur shared by 2 different frames. This is why halving it is important.)

And with 24 frames per second, you divide 1 by 24, get 0.041666~, halve that, and use 0.0208333~ as the shutter speed amount.
With 30 frames per second, 1 divided by 30 is 0.0333~, which when halved is 0.01666~.
With 60 frames per second, 1 divided by 60 is 0.01666~, which when halved is 0.008333~.
(However, there's an issue with Source Filmmaker not showing/caring about motion blur when the shutter speed is too low, and 0.008333~ is underneath that "minimum limit", meaning that you must settle for either no motion blur or "too much" motion blur when doing a 60-frames-per-second export. (An alternative is to make everything take twice as long, do a 30-frames-per-second export, and then speed it up in a video editor, but I personally wouldn't go through the trouble of doing that.))


Do note that motion blur does not get sampled across shot boundaries. As such, you don't have to worry about blurry camera swaps between shots, but you should also try to not blade a shot in the Clip Editor when you're not going to do instant camera position swaps in the second shot.

[Crashsune]

Awesome. Thanks, both of you!