Frame Rates are Tricky Beasts

Posted on by Larry

creativity

My goal in this article is to discuss the challenges in converting frame rates.

If everything you shoot, edit and output is a single frame rate, then don’t change anything. This is the ideal way to work. However, as you start to integrate elements that originate at different frame rates, frame rate conversion rears its very ugly head.

DEFINITIONS

Think of a video clip as a series of wooden children’s blocks connected by a piece of string. Each block represents a frame of video. As we pull the string, tugging the blocks along in a line, the frame “rate” represents the number of blocks (or images or frames) that pass an observer each second. Frame rate is measured in frames per second; “fps.”

Changing the speed of a clip is NOT the same as changing the frame rate.

This difference is significant. The first is easy, the second is hard.

We change the speed of a clip to create a visual effect. We change the frame rate of a clip to match the settings of our clip to the project. If you don’t need to match settings, don’t mess with changing frame rates.

SETTING BOUNDARIES

There are two sides to a frame rate discussion:

There is a lot of debate as to which is the “best” frame rate. Some feel that 24 fps is more “cinematic,” while 60 fps is more “real.” As you should know by now, there is no “best.” Just as there is no “best” car, camera, or restaurant; there are simply choices.

Converting to a 24 fps frame rate will NOT make your movie look “filmic.” It will, generally, just make it look worse. The “cinematic look” is a combination of: lenses, lighting, depth of field, shutter speed, shutter angle, motion blur and frame rate. Changing the frame rate only affects the frame rate, not the look.

There are no right answers, just louder voices.

Also, to keep this article to a manageable length, I will ignore:

These special cases don’t alter the basic rules of frame rates, though they can complicate understanding.

THE BASIC RULES

Whether you use Adobe, Apple, Avid, or any other video editing software on Macs, PCs or mobile devices, the basic rules of frame rates remain the same:

HOW WE GOT TO TODAY

In the early days of film, say 1890 – 1915, all cameras were hand-cranked. During this time, frame rates wandered from 8 fps to 30 fps, often in the same scene. In those days, the value of a camera operator was not based on their composition, but on the consistency of their cranking.

NOTE: This is one of the reasons comedies were so prevalent in the early days of film. Speed changes are inherently comedic and physical comedy does not require dialog.

As films grew in popularity and profitability, standards developed allowing cameras to be cranked by a motor, rather than by hand. Also, at this time, the industry settled on a frame rate of 16 fps.

Why? Because film was expensive and producers were, um, cheap. 18 fps provided the illusion of smooth movement without wasting a lot of film, and money.

This standard continued up until the advent of talkies which exploded on the scene in 1927 with The Jazz Singer. The problem was that 18 fps was not fast enough to support high quality audio. This frame rate yielded audio roughly equivalent to a telephone call.

So, a new frame rate standard needed to be developed – and the industry chose 24 fps.

Why? Because film was expensive and producers were still, um, cheap. 24 fps provided the illusion of smooth movement and relatively high-quality sound without wasting a lot of film, and money.

NOTE: Sound quality continued to improve over time, not by increasing the frame rate, but by shifting audio from an optical track to a magnetic track.

When video arrived, in the 1930’s, we had a major timing problem. How to get the TV receiver to “pulse” in sync with the transmitter? The solution was AC power. All across the US, power “pulsed” at 60 cycles per second.

Television engineers adopted this “universal” pulse as the basic timing circuit for video. Since video in those days was interlaced, where a single frame (complete image) was composed of two fields (a portion of the image consisting of all the odd or even scan lines), each field pulsed at 1/60th of a second.

Ta-dah! 30 fps video.

Except, over time it was discovered that high-voltage electricity “evaporated” from transmission lines when the cycle rate was too high. 50 cycles preserved more power over distance than 60 cycles (now called Hz). So, when much of the world was rebuilt after World War II, the utility companies, to save money and power, dropped the cycle rate to 50 Hz.

From there, the video industry derived 25 fps video, because interlacing was still in vogue.

So, at the dawn of the HD era in the early 1990’s, we had three principle frame rates: 24, 25, and 30 (which, with the advent of color was slightly modified to 29.97 fps, because why should this story be particularly simple?)

And, as we all know, with the rise of HD, our industry came together as a group and standardized on a single frame size and single frame rate.

– – –

Sigh… No such luck.

At last count, we now have nine different frame rates: 23.98, 24, 25, 29.97, 30, 48, 50, 59.94 and 60. (And, yes, 100 and 120 fps are knocking on the door. Please keep that door shut…!)

No WONDER we’re all confused. We’ve been handed a veritable Gordian Knot of frame rates!

CONVERSION OPTIONS

Frame rate conversion is the process of duplicating or removing frames such that, when the clip is played in a sequence that matches the frame rate of the clip, all action appears at “normal” speed. (I put “normal” in quotes because I couldn’t figure out an easy or good way to define normal.)

Most of the time, video editing software will automatically handle frame rate conversions. And, most of the time, I suggest you not worry about it, because, most of the time, it will look fine.

REMEMBER: “Camera-native frame rates always look better than converted frame rates.”

When it comes to frame rate conversions, there are easy options and hard options. Following the wooden block analogy I introduced earlier, we can’t just stretch frames to different rates because each frame is made of wood, not Silly Putty. Instead, we change frame rates by inserting or removing entire blocks.

EASY: 50 fps to 25 fps – or 60 fps to 30 fps

Assuming the video is progressive, conversion simply deletes every other frame.

If the video is interlaced, one field is deleted while the scan lines in the other field are duplicated. (Yes, this option reduces image quality. That’s one reason I hate interlaced video.)

EASY: 29.97 fps to 59.94 fps – or 25 fps to 50 fps.

Here every frame is duplicated. This does not create slo-mo because the video plays back at 50 fps, yielding the same movement as playing 25 fps video in a 25 fps project.

This does not significantly degrade movement quality, but movement will look more fluid if you shot 50 fps (or 60) originally.

MOSTLY EASY: 24 fps to 25 fps

The traditional way of converting 24 fps to 25 fps is increasing the speed of the 24 fps material 4%. This allows all frames to be displayed and, while the action is a bit faster, it isn’t so much faster that the audience will perceive it.

NOTE: Yes, this speed change means we need to speed the audio as well. There’s no free lunch.

HARD: 24 fps to 29.97 fps

This was done traditionally when converting films for television broadcast using a telecine.

Here, we need to create, essentially, six “new” frames every second. (The difference between 24 fps and 30 fps.) But we are dealing with children’s blocks here, we can’t create new images, we can only create new frames that contain existing images.

There are several ways to do this, depending upon whether you are working with interlaced or progressive images. The interlaced methods are quite complex and involve duplicating specific fields, not just frames.

But, here’s a simple method to illustrate the process. Take a group of four frames, then duplicate the last frame in the group. Over 24 frames this creates 6 new frames.

When played back at 30 frames a second, most viewers won’t notice the duplicated frame. However, for the discerning, your action will slightly stutter every five frames. This illustrates why you want to avoid converting frame rates.

HARD: 60 fps to 24 fps

Three quick reminders:

NOTE: Optical flow seeks to do just that, invent new frames. However, while good in theory, the results are often worse than not using optical flow.

Here’s an example of how this could be handled: We need to remove 36 frames from every second of video. Since both 60 and 24 are divisible by 3, we can divide each second into three “blocks,” or sections.  This means that a  20 frame block in the source clip needs to be converted into an 8 frame block in the destination clip. To do this:

As you can see, asymmetrical trimming (remove 1 frame, then remove 2, then remove 1…) gets us to the frame rate we need, but at the expense of potentially adding jitter to movement; say during an actor’s walk or as a car drives through a scene.  Whenever we convert frame rates asymmetrically, we run the risk of damaging the movement in the clip.

A SIDEBAR ON VIDEO COMPRESSION

Video can be compressed in one of two ways:

I-frame formats include: ProRes, GoPro Cineform, AVC-Intra and the DNx family of codecs.

GOP formats include: AVCHD, H.264, HDV and most formats that generate very small file sizes.

As I was writing this article, it occurred to me that camera-native GOP-format video will probably suffer more from image degradation as you change frame rates than video that was shot as I-frame media.

I haven’t tested this, and would like to hear other opinions, but if you are seeing blurry images when changing frame rates by small increments, I would suspect your video format is too blame.

SUMMARY

Frame rates are complex. However, a little planning ahead can simplify headaches. The best option – always – is to shoot the frame rate you need to output.

And, keep in mind, that a “film-like look” does not necessarily require a “film-like frame rate.”


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61 Responses to Frame Rates are Tricky Beasts

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  1. Robert Withers says:

    Thanks, Larry. I searched your article and the comments for “30 fps to 24” but couldn’t find it. People have told me it will look jerky and bad because frames will be deleted. Are there options?

    Also curious — it seems like DCPs are landing on a 24 fps standard, with exceptions. What do you think?

    Thanks,
    Robert

    • Larry says:

      Robert:

      Most theatrical films are shot at 24 fps, and DCP is the digital equivalent of film. Thus the tradition of using 24 fps.

      It isn’t a limitation of DCP, simply a carry-over of the standard frame rate of film.

      Larry

  2. Callum says:

    Great article.

    I’m exporting a TV commercial (used across multiple global markets) shot at 24FPS to 30FPS as is the spec required for Japan. I assume Japan TV must be used to this issue when dealing with anything shot at 24FPS?

  3. Silent film was actually 16 fps. The only film format using 18 fps was Super 8, which with its increased negative size caused it to be significantly superior to regular 8 silent film.

  4. Allynn Wilkinson says:

    Larry, I love this article! You explain an incredibly complicated topic and a very straight forward way! I only hope I can remember *half* of this the next time someone asks ME to explain frame rates!

    Oh… side note… the pioneer director Allan Dwan (1885-1981) explained that when he was a cameraman in the **very** early days of cinema he used to sing the Anvil Chorus to himself to keep his cranking even. Imagine what you’d have to sing to get 60 fps! Flight of the Bumblebee?!

  5. Larry thanks for explaining this topic so well. However there are ways to do the transcode from frame rate to frame rate using Cinnafilm’s PixelStrings look at PixelStrings.com. Cinnafilm has a whole suite of tools that can transcode between frame rates and do all kinds of scaling and Denoise, deinterlacing and retiming. Quite a remarkable product and now in the cloud with PixelStrings.

  6. William Hohauser says:

    I encounter frame rate issues as I frequently provide mixed content programs for projection in theaters across the country. One issue is that theaters never have the same projection equipment as each other. The best equipped theaters have DCP, BluRay and some form of direct playback from computer. PlaybackPro is a good program for that.

    For one regular job I receive 25, 25 and occasionally 30f shorts to be combined into single programs. The solution I use is to create 30p BluRays as 24 and 25 play fine in 30p. Unfortunately some theaters don’t have BluRay, just DCP which in this country is fixed at 24f despite 25, 30, 50, 60 and 120f are available possibilities. European theaters tend to be more liberal in accepting different DCP frame rates although I have never delivered DCPs in those frame rates. Regardless 30p to 24p doesn’t work very well, especially with live action. No matter what you do, smudging frames together looks as odd as dropping frames. Fortunately people in this country are creating theatrical content in 24 or 23.97pnow so I infrequently get 30p/60i shorts anymore. Unfortunately in Europe, many shorts are created for television so 25p is delivered instead of 24p. Slowing down content isn’t always an option but 25p usually drops into 24 with minimal issues even with live action.

    Another job I do is creating film history programs that include tons of historical clips from all sorts of sources. Usually these sources are old SD telecine transfers at 60i. These look horrendous converted back to 24p for DCP and I’m not talking about the resolution. Drop down telecine techniques are discussed here but the complication is which technique was used for a particular clip. They are different where the frames are duplicated. Rather than spend hours analyzing each clip I deliver these programs at 30p for playback from a computer. Works great. It’s worth mentioning here that even though a computer can play any frame rate, professional projectors can not. The projectionist has to prep the projector before the program for a specific frame rate and resolution. This is why I deliver programs where everything has been conformed in post. The projectionist can start the program and sit back without worrying.

  7. Avdshare Video Converter can change almost all kinds of video format frame rate, including AVI, VOB, MP4, MXF, FLV, AVCHD, MOV, SWF, MKV, etc.

    • Larry says:

      DaveClark966:

      You are correct. In fact, Compressor, Media Encoder, even ffMPEG can change frame rates. The issue isn’t HOW to change frame rates, but what happens when you do.

      In virtually all software, including AVDShare, you are either stretching the duration of an existing frame, or dropping frames. You aren’t creating new media.

      Larry

  8. Ghadir says:

    Hi Larry, Thank you for this great article, I’v worked on a project for 6 months which I shot in 4K resolution and 30FPS. i have only few hours left for the deadline and the client is telling me they only accept 25fps. What is the way to convert from 30fps to 25fps without the minimum trouble possible to tweak things around? its a landscape project and entirely shot from a heli using t he same camera and same frame rate.
    I use final cut pro X. I really appreciate your input and help.

    • Larry says:

      Ghadir:

      First, finish your project and export it from FCP X. Because this is an interim file, you might consider exporting it as ProRes 4444, which is totally lossless, but at a minimum export the project and match your project settings. DON’T!! change the frame rate of your project file, it will change all your edits!

      Then, use Compressor to convert the frame rate from 30 to 25 fps and convert it into the video format you need to deliver. What frame rate conversion will do is drop every fifth frame, but this is much better than not delivering your project.

      Larry

  9. Oz Ikiz says:

    Hi Larry,

    Great article.

    I guess my question is if I shoot it at 60fps, do I always have to use it at 24fps if my project is at 24fps? Sometimes I find some of the scenes are very slow so can I just slow them less than 40% without looking weird? Or do I always have to slow it down to my project frame rate? Thanks

    • Larry says:

      Oz:

      In general, you should shoot at the frame rate you want to deliver.

      If you put a 60 fps video into a 24 fps project, it will drop 2 frames out of 3 – approximately – automatically. You don’t need to slow anything. The NLE will handle it.

      Larry

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