Quick guide to video frame rates and sizes

By | 16th October 2013

Front panel of Teranex 2dVideo can be shot, edited and delivered in more shapes and sizes than ever before. In this short guide we’ll try to explain frame size, frame rates, interlacing and progressive scan in plain English.

What the numbers with letters at the end mean

You’ll see numbers like 720p or 1080i used to either describe a camera specification, a video file or an option when saving a video. The numbers relate to the height of the picture frame in pixels:

480 = 720 x 480 pixels and is normal for NTSC type standard definition (SD) video.
576 = 720 x 576 pixels and is normal for PAL type SD video.
720 = 1280 x 720 pixels and is a high definition (HD) format.
1080 = 1920 x 1080 pixels. This is also high definition and is usually described as Full HD.

2K, Digital Cinema, 4k and UltraHD bring a whole new combination of frame rates and sizes – we’ll stick with HD video resolutions for now.

The letter at the end tells you whether or not the video is progressive scan ( p ) or interlaced ( i ):

p = Progressive scan. This means the picture is recorded in full frames in one go.
i = Interlaced video. This means the pictures is recorded in alternate odd and even scan lines or “fields”.

Online content is always progressive scan. If you shoot interlaced then you will need to de-interlace the content at some point for online delivery. This is usually best left to the end of the edit at the rendering stage especially when the content is being sent to multiple destinations e.g. Blu-ray, DVD, broadcast and online.

Most standard SD video and many HD recordings including broadcast TV are interlaced. Using interlaced images has a couple of advantages. It reduces the amount of data needed to store or transmit the video. The alternate lines (fields) are recorded sequentially so with a moving subject each field shows a slightly different image. When interlaced video is viewed on screen the human eye doesn’t notice the image is based on fields and is fooled into blending them into full frames thus creating smoother looking motion at lower frame rates.

Old TVs with a tube (CRT screens) were all interlaced. Modern flat screen TV panels are progressive scan. You can play interlaced video on progressive scan TVs. There’s no need for the viewer to adjust anything because the TV can work out what to do with the image (de-interlacing). Computer graphics card adapters and displays are progressive. Interlaced footage viewed on computer displays can make the footage look jagged or coarse if the software cannot de-interlace the files correctly. If you are editing video for interlaced delivery then you should preview the footage on a proper video output with editing hardware such as a Matrox or BlackMagic card to ensure that what you see is what you are actually going to get.

Frames rates? What’s all that about?

The frame rate of a video describes how many times per second the video picture is updated. The higher that number, the smoother the motion in the video will appear to the human eye. Video with lower frame rates can, in some circumstances seem to flicker or judder especially during motion such as when the camera pans across a scene.

The reason we ended up with different frame rates in different parts of the world is because TV cameras, TVs and everything else in the broadcast chain were connected to the mains supply. Here in the UK the mains supply runs at 50Hz whereas in the other countries the mains runs at 60Hz. It therefore made sense to base the refresh rate of TV equipment around the frequency of the mains supply for any given country. In 60Hz countries they started out using 60Hz for TV but when colour pictures came along they slightly altered the refresh rate to to 59.94Hz for technical reasons. The odd number creates a bit of confusion – 59.94Hz is often rounded up to 60Hz. Half of 59.94 is 29.97 but is often rounded up to 30 for convenience.

These are some common frame rates or “frames per second” (abbreviated to fps) and where they usually apply:

24fps – this is the frame rate of film. In video it tends to be 23.976fps that’s used to mimic the “film look”.
25fps is 50Hz halved & used in SD or HD. Usually PAL countries like the UK, Australia and most of Europe.
29.97fps is 59.94Hz halved – Used in SD or HD. Usually NTSC countries incl. USA, Canada, Japan.
30fps – Some cameras do actually record 30fps but often it might really mean 29.97fps rounded-up.
50fps – Used for HD recordings in PAL countries – gives a really smooth motion pictures.
60fps (usually 59.94 rounded-up) – Used for HD recordings in NTSC countries – again gives smooth motion.

Joining the numbers, letters and numbers together

Now we know what the different bits mean we can hopefully understand the full thing easier. Here goes:

720p/50 = Frame size 1280 x 720, progressive scan with 50 frames per second.
1080p/25 = Frame size 1920 x 1080, progressive scan with 25 frames per second.

With a bit of luck that makes perfect sense. It would be great if things were this simple and when describing progressive scan video it pretty much is that easy.

We’ve already explained that interlaced video images contain alternate lines or fields. Each field contains half the information needed to draw a full picture. The numbers for frame rates should only describe full frames. So an interlaced video with 50 fields (or half frames) per second actually only has 25 full frames of information per second. Hence it’s written as 25.

1080i/25 – Frame size 1920 x 1080 Interlaced with 50 Fields Per Second.
480i/30 – Frame size 720 x 480 interlaced with 59.94 fields per second.

It’s worth noting that all 1280 x 720 video is progressive scan – there isn’t an interlaced version.

Forward slash frame rate

You may see video described as 50i. Most people would accept this is 50 interlaced fields per second but when it’s shown in context next to a frame size the EBU preference is to always describe the frame rate 1080i/25 and NOT the field rate.

So 1080i/25 does mean exactly the same thing as 1080i50 that you may see elsewhere. The forward slash in the middle indicates that it’s EBU preferred terminology.

Odd sizes and frame rates

Most video cameras and TVs show a landscape picture. The picture is wider than it is tall. But these days, especially with mobile phones video can be shot in portrait mode so the image is taller than it is wide. So when we shoot a full HD video portrait the picture might be 1080 wide x 1920 tall at 30 frames per second. It’s still a 1080p/30 video.

Some recordings, such as in HDV video camcorders, and many HD TV signals are stored or broadcast at 1440 x 1080i. This is another bit of trickery used to reduce the amount of data needed to record or broadcast the footage. The pixels in the video are oblong (non-square). The pixel aspect ratio is 1.33:1. Essentially this means the pixels are stretched horizontally to fill a 1920 x 1080 frame during playback. This is hardly noticeable to the average viewer. You may also see 960 x 720 pixel video from some cameras. Again these employ non-square pixels to generate a 720p image.

Sometimes cameras can run at low, variable or selectable frame rates. An example of this might be a low end web cam that might run at 15fps because that’s good enough for a talking head shot or to fit in with a slower internet connection. On a faster connection the frame rate might be increased to 30fps. Note: The camera itself possibly runs at 30fps all the time but for technical reasons it’s picture is only recorded or transmitted at lower frame rate.

It’s sometimes necessary to deliver progressive video with equipment that expects (or was designed for) interlaced video. This is achieved by using Progressive segmented frame or PsF. It’s similar to interlaced but the two fields are identical.

Very high or low frame rates are employed to produce a special effect such as ultra-smooth slow motion or time-lapse videos.

We hope you find this quick guide helpful and easy to understand. Feel free to ask a question or comment below. We’ll do our best to reply.

12 thoughts on “Quick guide to video frame rates and sizes

  1. james

    Thanks. It is very helpful

    I need to do a recording with 1080i camera with 1920×1080 frame size with 25fps. Is it possible in 1080i?

    which camera is useful?

    Reply
    1. Gavin Gration Post author

      Hi James,

      1080i cameras record 50 fields per second. You can de-interlace the image to make 25p.

      Nearly all modern video cameras (PAL UK models) can do 1080p. Some but not all will do 1080i as well. In essence the choice of camera depends largely on how much you want to spend and what type of activity you want to film.

      Video cameras with smaller image sensors are better when there’s a lot of action, moving around, longer duration events etc. Large sensor (DSLR type) cameras are more suited to relatively static subjects and will give a different ‘look’ than a video camera. This isn’t a fixed rule – some DSLR cameras now have fast AF and servo zooms that rival traditional video cameras. Likewise there are now video cameras with larger sensors that give you some of the benefits of both. Most large sensor cameras don’t do 1080i (usually 25p or in some cases 50p).

      Reply
  2. martyn moore

    Interesting, Gavin. Thanks for that. So when you guys go out to shoot on location, say, at an office or factory, what frame rate do you mostly shoot at? I’ve shot at 1080i for years but never really knew why!

    Reply
    1. Gavin Gration Post author

      That’s a very fair question Martyn. 1080i was and can still be a great shooting option for some cameras. The interlacing achieves smooth motion at a low 25fps (50 fields). A lot of cameras such as the EX1/3 and PMW350 could only achieve either 50i or 25p at 1080. For fast action such as sports or a live event 25p could produce stuttering with horizontal movement so 1080i or 720p50 was preferable. For most web based jobs to date we have used 1080p25 or 720p50 but our latest cameras are capable of 1080p50 so that’s what we use now. The online world is also offering higher frame rates with YouTube starting to show content at 1080p50 and 1080p60 in the last couple of months.

      Reply
  3. dean

    Hi. This all makes so much more sense. Thank you very much.
    I was just wondering about file size…
    If I shot 1 min video at 1080/25 and then exactly the same scene again for 1 min at 1080/50 would the 50 frames file be twice the size?
    What I mean is 25frames be 200mb and 50frames file be 400mb? Or does it not work like that?

    Reply
    1. Gavin Gration Post author

      Hi Dean,

      Thank you for your question. The file size is dependent on the data rate of the file rather than the frame rate.

      When recording in camera you will have access to different data rates in megabits per second (Mbs). Sometimes described as a quality setting such as SP or HQ. The flexibility to select a specific frame rate or data rate combination varies from camera to camera.

      Common bitrate options on professional cameras include 35Mbs, 50Mbs and 100Mbs. However, not all frame rates can be recorded at all of those data rates. e.g. 35Mbs works with 1080p25 but 35Mbs may not be an available option for 1080p50.

      In theory 1080p50 at 100Mbs would produce double the file size of the same content at 1080p25 50mbs if your camera allowed those options.

      Does that help?

      Reply
  4. mini

    Hey!

    recently i downloaded a few videos from internet. few were having frame rate of 25 fps while some were at 29 fps.
    Howcome 29 fps? is it a standard for internet videos?

    Reply
    1. admin Post author

      29.97 is used in USA/Japan. Many webcams and most mobile phones globally default to 29.97 or 30.00.

      That’s primarily the reason why 29.97 is so widely seen online.

      Reply
  5. mary

    Hi,

    Novice here.
    I’m wanting to use quicklime to shoot 720p/30 on my mac, and my two iPhone’s as cameras, also 720p/30. But my Zoom HQ8 camera only shoots 720/25 @ 8MBS 0r 720/50@ 16MBPS (that is when its switched to Pal. Im in Austrasoia)

    When I switch it to NTSC it records 720/60 @15mpbs, or 720/30 @8mbps

    However when i look at the specs on the footage from my Zoom, Final Cut says it is 720/29.97. Is this close enough to edit? Or is it a problem to edit NTSC with iPhone video?

    Reply
    1. admin Post author

      Hi Mary,

      I’m not familiar with the Zoom HQ8 camera BUT it’s likely that it is shooting 720p60 (that could actually be 59.94 interlaced or progressive).

      Final Cut is usually fine with various fame rates – especially from NTSC sources.

      Often kit spec says 30fps but it’s really 29.97fps or vice-versa. Again FCPX will happily edit the lot on the same timeline with no ill effects.

      One thing you need to watch is that your project settings (timeline) must have a frame rate of 29.97 or 30fps.

      The default in FCPX is to set project to match first piece of content. You can over-ride this and enter the settings you want.

      iPhone footage is often variable frame rate – it might be identified as 29.97 or 30 but it may not be constant. Depending on the camera (front/rear), the device model and portrait/landscape filming mode some units record at either 24fps OR 30fps. Both are compatible with NTSC 29.97 insofar as FCPX is concerned.

      I’m confident you won’t have any problems but to test that I recommend making a short, simple edit from all your sources and export that to a file. See how the edited file looks.

      Reply
  6. gdpinchbeck

    iphone 6 movies are not the format or size suitable for video editing (PPro cs4 adobe encoder does not work) can I purchase file converters to avi or other suitable formats at 4/3 ratio, is this possible? Are you able to recommend?
    David

    Reply
    1. admin Post author

      Hi David,

      iPhone 4 records 16:9 aspect ratio 1280x720p video via the main camera at a nominal 30 frames per second. However, Apple devices tend to record at an irregular frame rate which causes Adobe Premiere to choke on the files.

      The quickest fix for making iPhone videos compatible with Adobe Premiere is to run the file through Handbrake Website.

      Choose one of the 720p30 presets (or the iPhone/iPod preset). This will convert the .mov from the iPhone into an MP4. The MP4 file should work fine inside Premiere.

      If you’re using the front facing camera then it will be 4:3 aspect ratio 640x480p up to 30fps – again the frame rate will be variable. You can run these through Handbrake too.

      Handbrake Screenshot

      We’ve found Handbrake to be very useful. It’s completely free and safe in our experience.

      Gavin

      Reply

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