Can you really convert from 30 to 24 fps?

[Charlie McCarrick]

Can you REALLY convert 30fps to a true 24fps? 24 frames per second doesn't just mean less frames per second, it also means that each frame is a little longer than those of footage shot at 30. But when you decrease the frame number, you can't extend the length of the frames that have already been shot, unless you slightly slow down the footage. Is this how it's done? Thanks.

[Rob Lohman]

The frames are not longer. They are displayed longer. If you can
project your 24 fps footage at 24 fps then, yes, it will display
longer as well. BUT, if you use a normal video projector or TV it
will not be the same. BUT, the motion signature can be different
which is what matters most.

The best way to acquire the 24 fps look is to shoot at 24 fps. The
presentation is less important (look at DVD on your TV for example).

Converting from 30 fps will yield true 24 fps, but it won't look
exactly the same due to it being converted. As always, try and
test yourself to see if you like it good enough.

[Charlie McCarrick]

The frames are displayed longer? Interesting, but how does this work? I understand how a TV will shorten the displayed lengths of frames from 24 fps footage. And I now understand how that would alter the film motion. But what about the other way around?

What if I shoot at 30 fps, then convert to 24 fps, and project the revised footage at 24 fps? How are the frames displayed longer? Basically, how does the conversion extend the displayed lengths of frames? Are parts are the frames frozen to make the frames last longer and fill up the extra time? Are the frames slightly slowed? Or is the extra time filled with blank spaces at the end of each frame, which you can't actually see? I downloaded the Magic Bullet demo to see how I like the conversion, but I'm still curious to know how it actually works. Thanks.

[Don Donatello]

30 fps is converted to 24fps by dropping 6 fps ..
you can play the 24fps on your computer.
fps is just a speed ..X frames are shown in 1 second.
remember 24fps to 30 is done by adding 6 frames a second (it repeats fields/frames)

if you attempt to play out to a NTSC TV at 24fps from your NLE most will ADD pull down ( adds 6fps by repeating fields to =30fps) because NTSC is 29.97 fps ( call it 30fps) ..

if you have a digital projector it may or may not be able to run at 24fps ? most add pull down to get it back up to 29.97 ...

on 24fps DVD's the DVD player will add pull down to get it back up to 29.97 ...

if you have a HD plasma made in past 1 -2 years many will play back 24p material at 24fps ... .. but for standard def tube TV's you are going to be viewing at 29.97 ...

[Rob Lohman]

That's easy to answer Charlie. You need to think differently.
If you have 24 frames displayed at 24 fps you are seeing 1 second,
right? If you have 30 frames displayed at 30 fps you are also
seeing 1 second, right?

So:
1 / 24 = 0,0416 seconds per frame
1 / 30 = 0,0333 seconds per frame

In other words, each frame in a 24 fps second movie is being
displayed longer then when you are watching 30 frames per
second. Which is logicaly since the amount of frames per second
changes and so does the exposure or time a picture is up.

So if you convert 30 fps to 24 fps those *resulting* 24 frames
in a second will stay up slightly longer. Ofcourse the *original*
frames from your 30 fps movie do change temporally in regards
to the 24 fps movie. How depends on the algorithms etc.

[Ken Hodson]

Algolith and Twixtor are two programs that can help.
Ken

[Alain Aguilar]

Has anybody seen any practical or positive results by converting from 30 to 24 fps?

[Rob Lohman]

The only real use I can think of is when you want to put your
movie out on film (for distrubution)? Since all other forms will
probably suffer from the conversion. But that's just an "opinion"

[James Ball]

if you are wanting to go out to film with video originated at 29.97. PAL video can be converted without dropping frames. For a film look on video people often shoot 30fps progressive.

As others say 24fps progressive is designed for giveing a more filmic look and to allow indies the opportunity to go to film easier if their film takes off.

Even so, some very nice transfers have been made from DV NTSC and PAL to film.

If you wish to "convert" to 24fps from NTSC video:

1. shoot 60i. Same as 29.97 interlaced.
2. Deinterlace and separate the frames and double the horizontal resolution to make 60p.
3. Use Twixtor, ReTimer, etc. to convert to 24fps.

This is the general route that post houses take to convert NTSC to film.

[John Heskett]

I could never really figure it out. So, I asked Ben Waggoner, the author of Compression for Great Digital Video. Ben stated that in order for it to be perfectly smooth in playback, the closest you could get was 25fps in NTSC. In the end, I just took Ben's word for it and converted to 25fps.

[Doug Turner]

I'm in PAL land, I 'convert' to 25fps from 50i using Magic Bullet - and this looks filmic to me.

I can see no reason why NTSC people can't 'convert' to 24fps from 60i (interlaced 30fps). I believe Sony Vegas and Magic Bullet do this quite well...

All Magic Bullet (and similar plugins/apps) does with 50i footage is to create 2 identical fields 25 times a second - so although their output is still 50i, it looks 'progressive' merely because adjacent fields are identical!

There is no perfect answer, just do what looks better - you can theorise until the cows come home... experiment :)

[John Heskett]

We are only talking NTSC here PAL is altogether different.

Ok, here is the quote form Ben Waggoner:

"Your output frame rate should be an integer division of your source, so if you're coming from 29.97 fps source, you should be doing an output frame rate of either 29.97 or 15 (well, 14.98). 24 would give you uneven motion."

I'm no math brain and to my simple math I could divide 30 by 6 and I could divide 24 by 6 so I thought it would work. But I was wrong and division is not the same thing as an integer division.

What I did was find an integer table and found out that 25 is an integer with/of 30. What systems (software or hardware) do to get to 24pfs is subtract or add frames to make up the difference. If you don't want these subtracted or added frames to mess with the final edit or audio sycn then you have to stay with an integer division of your source.

[Barry Green]

That's not what's happening at all. First, NTSC is not 30 "frames" per second, it's 60 fields per second (rounded off).

So to simulate 24 frames, the NTSC fields get blended together, at a ratio of 2 fields -> frame 1, 3 fields ->frame 2, 2 fields-> frame 3, 3 fields ->frame 4, etc.

So the pattern is 2:3:2:3:2:3, etc.

The result is that 10 fields get converted into 4 frames, and if you multiply that by 6 you get 60 fields converted into 24 frames.

No frames are "dropped" -- that would result in jerky, unwatchable footage. By using the field-blending technique, a good amount of resolution is maintained, nothing is dropped, and the results are generally viewed as "okay". That's what Magic Bullet does, that's what all NTSC->film transfer houses do, etc.