Slow motion - best practices

[Kyle Kelley]

Hi all,

I'm shooting a short in a few weeks on the Scarlet and am looking for some advice on shooting some slow-mo shots. The majority of the film will be in 4K HD at 24fps, but there are a few important moments that I'd like to capture at high speed. I was hoping to be able to do some tests this weekend, but got swamped with another gig and am working straight through up until we start production on the short. Any help with the following would be greatly appreciated!

• How does 4K HD @24 fps slowed down using a utility such as Twixtor compare with 2K footage at 60fps, or 1k at 96/120?
• Is it completely necessary to blackshade when switching between 24fps to 60, 96, 120, and maintaining a 180 degree shutter angle? From what I've read, it's recommended to blackshade for any significant frame rate or shutter speed changes, but the 15-20 minutes it takes to do this is a pretty major drag on set. Of course, we could schedule around it, but I'd love another option if possible.

I realize the Scarlet isn't the most desirable option for high speed cinematography, but it's what I have to work with. Any tips would be fantastic!

[Demetri Zuev]

I shot almost 2 hours of Scarlet 120fps footage today.

Here's what I learned:

— Close your aperture. Most lenses are at their best 2 steps down from wide open. This is really critical when shooting 1K on Scarlet — because of the resolution you may get artifacts wide open and 2-3 steps down from wide open (it all, of course, depends on a particular lens). My working f-stop was 5.6-8.

— Get a wide lens. And I mean a really wide one. If you're planning on doing any sort of even medium shots at 120fps 1K you should get something in the range of 17mm and up. Crop factor here is 6x (when using still glass), so go figure. My Nikon 17-35mm wasn't wide enough for me today, I would prefer something like 8mm.

— Shoot lowest compression. 1K WS can be shot at 3:1, you want to get as much detail as possible.

— Look for "bleeding edge" sharp lenses. Again, because of the resolution, you can't compensate by downscaling here, you record at 720p, you get 720p, that's it. So the sharper your lens the more detail you will get in a shot.

— Work with 1K footage in post. I see a lot of test videos showing 1K Scarlet footage and almost all of them are blurry. Use unsharp mask, use denoise tool. Your dslr as well as any codec-based camera does all that internally before you get a final file out of it. You shouldn't forget that RAW footage isn't always good to be used "as is", it's supposed to be corrected in some way.

— Pick your lights carefully. If you're shooting with anything other than the good old Sun as your light source, you should be a 100% sure that your lights won't flicker when you'll be shooting your 120fps footage at 1/240 shutter speed (following the 180 degree shutter rule). ARRI has some lights that don't flicker right up to 1/750th if I remember correctly. Also remember that by closing your aperture to something like f5.6 and setting your shutter speed to 1/240th of a second you're cutting a huge amount of light from your shot, so your lights have to shine bright.

I wouldn't go a "twixtored 24p" route. You can always see artifacts here, if you look closely. 2K 60p seems a more likely option if you want to give it a try, but Twixtor, as any other plugin requires a lot of setting adjustment for your particular footage in order to get you to your desired destination. If you go a 2K 60fps route still set your shutter at 1/240th, it would be easier for Twixtor to process.

[Maik Müller Menendez]

All you need is a sharp lens thats wide enough to compensate the higher crop-factor, more light to get proper exposure with the higher shutter speed and always use the best possible compression-setting. That's all.

Slowing down with software like twixtor is a option, but the results heavily depend on source-material, how much motion and small details are in it and so on... I wouldn't trust on it as single solution, especially without having done tests before production. You should try to get multiple takes to have more options in post. I would probably shoot the scenes in 4k@24, 3k@48 and 2k@60, using Twixtor with the 3k-version to get it a bit slower could be interesting i think. I'm not a Twixtor-expert, but i have seen many scenes that have been slowed down by software in which the interpolated frames destroyed the 180degree-look.

As for blackshading, i did my tests and so far, i think it isn't that extremely essential. I would do it once before a big shoot, but not everytime i change resolution/framerate.

[Jon Shaw]

Interesting discussion, I have used twixtor a fair amount and had varying results, if you are shooting something with a plain background, not masses of detail then it works well. If its a complex scene I reckon it looks pretty average.

[Jon Carr]

+1 on using a wide lens

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[Jaakko Rinne]

How does 4K HD @24 fps slowed down using a utility such as Twixtor compare with 2K footage at 60fps, or 1k at 96/120?

Like the others have pointed out, the end result with Twixtoring will depend greatly on the footage. You can help the plugin along with masks and guide paths, but I'd test it before committing. You should also remember to set your shutter speed according to what you intend the end result to be. If you want it to look like it was shot at 100fps, set your shutter to 1/200.

2K and 1K will obviously be softer than 4K and if you're mastering to 2K or higher you will always get a softer image if you're shooting at a lower res than 3K. That being said, numerous films have used 2K footage from the Red One without issue. It's really up to the requirements of the production if that softness is an issue or not. And obviously 1K 120fps is WS, so it'll be a different aspect than 4K HD.

As was suggested, a good option might be to shoot 48fps 3K HD and Twixtor that. It also helps to have a bit of extra resolution with Twixtor for cropping, as it's almost guaranteed that there will be artifacts near the edges of the picture (if the image content near the edges is changing).

Is it completely necessary to blackshade when switching between 24fps to 60, 96, 120, and maintaining a 180 degree shutter angle? From what I've read, it's recommended to blackshade for any significant frame rate or shutter speed changes, but the 15-20 minutes it takes to do this is a pretty major drag on set. Of course, we could schedule around it, but I'd love another option if possible.

It isn't completely necessary. I've switched between 25fps and 120fps regularly while shooting in daylight at 320 ISO without stopping to blackshade and haven't had any problems, but I would definitely blackshade if I had to use a much higher ISO.

EDIT to add: Also a +1 for using a sharp, wide lens for the lower resolutions and for not shooting wide open.

[Mike P.]

Very interested in this as well; with AE/pixel motion/Twixtor, I've get decent results slowing down by 33-50%... e.g. 60fps slowed to 90fps then ran at 24fps still looks great. My first test with this would be 3k/48 at RC3:1 slowed to a 72fps-equivalent via software (300% real-time at 24fps playback rate)... I'm hopeful that the differences/artifacts at those settings will be negligible.

[Jaakko Rinne]

Very interested in this as well; with AE/pixel motion/Twixtor, I've get decent results slowing down by 33-50%... e.g. 60fps slowed to 90fps then ran at 24fps still looks great. My first test with this would be 3k/48 at RC3:1 slowed to a 72fps-equivalent via software (300% real-time at 24fps playback rate)... I'm hopeful that the differences/artifacts at those settings will be negligible.

With 50% speed only every other frame is Twixtored so any artifacts are visible every other frame, but with a speed of 10% 9 out of 10 frames are Twixtored and when there's artifacting it'll be on screen for 9 frames. What you want to do for extreme slow motion with Twixtor is to build your edit around the frames that work.

[Kyle Kelley]

Thanks for all the advice, folks. Time permitting, I'm going to do a few tests this week, and assess those results in combination with your suggestions to make a decision for production. Either way, I'll post the results on Vimeo for anyone who's interested.

[Sergio Perez]

My experience with the Epic at 400fps (not scientific, "Philip Bloom" style personal experience perspective)

1- Get as much light as possible. Its preferable to Overexpose than to underexpose, or expose the scene for a brighter scenario. Do NOT go beyond 320 ISO when setting up exposure. Since its basically cropping the sensor its also zooming in on all noise and compression artifacts. From my experience, raising the iso from a slightly underexposed 320 1K shot equals very noisy footage.

2- If you're using Redcine X for transcoding, don't be affraid to mess around with the unsharp mask tool. Essential, by the way.

3- If you're in a PAL country, 100fps is the best you can get under "normal" street lights. From there on you need specialized lighting.

4- As others have said, record in the lowest compression setting available for High speed shots.

I think this is all I can add to the already mentioned points in this thread.