16mm 18fps to video 23.98 fps

I'll be grading some 16mm films that have been scanned to video at 18fps. What are the best practices for ending up with a graded video file at 23.98 or 24 fps that runs at the correct speed?

Not an easy task.
Set motion estimation to AI, drop 18fps into 23.98/24p timeline and hope for the best
There is not much you can do expect fps conversion, which can always have some artefacts.

18 fps is common for 8mm film but not 16mm as it was nearly always 24fps. Might be worth checking if the scan was correct.

What do you want to have happen with the frames between 18 and 24?
What Andew is suggesting is a blend of the real frames, either with Frame Blending or Optical Flow, which I am not a fan of.

I would suggest to have Frame Interpolation set to Nearest Neighbor, so the frames are duplicated instead of calculated.

I never suggest blend or repeated/removed frames methods for real content as this is poor way.
Repeated frames will look jerky.
Blended will look crap- ghosting, double edges...
Conversion is the only way. You try different algorithms and choose the best result. Today's algorithms are not perfect, but very good. Make sure you have timeline with cuts (if needed run cuts detection). At the end you can also choose best method for given scenes and then join all together at cuts for even better end result.
There is open source tool (FrameRateConverter) which switches by itself to different interpolation for different scenes.

I never suggest blend or repeated/removed frames methods for real content as this is poor way.
Repeated frames will look jerky.
Blended will look crap- ghosting, double edges...
Conversion is the only way. You try different algorithms and choose the best result. Today's algorithms are not perfect, but very good. Make sure you have timeline with cuts (if needed run cuts detection). At the end you can also choose best method for given scenes and then join all together at cuts for even better end result.
There is open source tool (FrameRateConverter) which switches by itself to different interpolation for different scenes.

Set motion estimation to AI

That's blending

Anything with only 18 real fps will be jerky, hence the question: "What do you want to have happen with the frames between 18 and 24?" (I know Andrew is not the original poster)

By AI I meant OpticalFlow with use of AI engine for motion estimation.
Resolve GUI is bad as I have no idea what motion estimation setting have to do with Nearest option They should be greyed out when you choose Nearest or Blend.

With interpolation it won't be jerky - this is the whole point of interpolation, compared to repated/removed frames method. Even simple blending is not jerky, but looks bad.

Thanks for your replies. As it turns out, the 18-->23.98 conversion will be handled by the lab that did the scans, who are purported to have lots of experience with this.

Re the frame rate, these are silent art films, all shot with a spring wound Bolex, and I've been told the fps = 18.

Again, your thoughts are much appreciated.

these are silent art films

Like those of Nathaniel Dorsky, for example, @18fps?

Anyway, the answer is as above: the best you can do is experiment with Resolve's motion toolkit to see what works best for each clip: optical flow, speed warp, enhanced, nearest, etc. In the old days the silents shot between 16 and 18fps were converted to 24pfs simply by duplicating frames, but with predictably jerky results. Probably not want you want.

But there's no way 18 goes into 23.976/24, and the conversion methods available today don't produce flawless results, which amounts to creating frames which never existed.

18 fps is common for 8mm film but not 16mm as it was nearly always 24fps. Might be worth checking if the scan was correct.

This.
I would just change the clip attribute to 24fps and see if it plays correctly in a 24fps timeline.

16mm 18fps was common in two realms: home movies, and non-narrative silent films referred to as "avant-garde" or "experimental".

Based on the OP's report of the material, 18fps is not surprising.

I wonder if you might have better luck trying 29.97 just because its closer to to a 2:1. Having shot a lot of bolex footage like that in my youth, I wasn't even always sure what speed the projector was going at nor did I particularly care, so if you just treated it as 2:1 the speed difference might not matter . No idea if that would transfer any better though. Just a thought . Back in those days I think the black gap in between the frames added to its feeling but again I have no idea how that would look in video. As I recall ( and I could be wrong), sometimes the projector blades would actually give you 2 brief images of the same frame.

Best way would be to show it at native speed, but with eg. 4x repeat so at 72Hz. Projectors do support 72Hz, as this is how 24p is displayed (3x repeat).
You could also show it at 50Hz (3x18=54), so then speed difference is 8%, so not that high for this type of content.

What is the reason that the footage needs to be 23.98? It’s not going back to film and I’m assuming theatrical release isn’t the end game either. At 60 fps you could triple the frame rate to 54 then have every tenth frame be an interpolated frame. Wouldn’t be any worse than a 3:2 pulldown.

Good Luck

It doesn't work like this.
When you interpolate about all frames need to be changed as they have to be in new 1/fps second position. Some frames may stay original, but most need to be "made-up".
Same if you do eg. 24p to 25p conversion- it's not one frame which is created. All are new as they need to fall into 1/25 of the second time (which is different than original 1/24). I had same reasoning as you long time ago until I was corrected by Snell engineer