Back to Basics: A Primer on those P's

If you’ve been researching high definition filmaking and are relatively new to the game, you’ve noticed terms like 24p, 50p, 720p, etc. What’s up with all the “Ps”? And what’s the difference between 24p and 720p? Well, I hope this article will shed some light.

P IS FOR “PROGRESSIVE’

No, it’s not progressive as in the political sense. Progressive relates to how the video image itself is created. Traditional video is interlaced. That means it is comprised of 60 alternating, interlaced fields (thus the term 60i which you may have come across too). When sped up at the traditional North American frame rate of 29.97 frames per second (also simply referred to as 30 frames per second), each group of of those 60 fields creates one image to the human eye.

Image from Iowa college of engineering. http://www.icaen.uiowa.edu/
Image from Iowa college of engineering. http://www.icaen.uiowa.edu/

A progressive image is one where the frame is one complete image, as opposed to interlaced lines. Progressive video is one of the ingredients of creating that all-desirable “film” look. One other key ingredient is the frame rate. (Note: there are many factors that go into creating a true film look. International DP Philip Bloom has a free, 54 minute audio interview about creating the film look you can get just for signing up on his F-Stop Academy website. Do it.)

24

Besides being a popular show on Fox, 24 is also the number of frames per second (fps) that filmmakers like to use in order to make their productions look like traditional film, since that is the frame rate of film. So, when you see the term 24p, that means progressive video that is 24 fps. 25p is progressive video at 25 fps (the frame rate for PAL video which is used in Europe and some Asian countries).

Other popular frame rates include 60p and 50p. The benefit of these higher rates is the ability to create “true” slow motion. Typically, to get slow motion with video, you have to slow the video down in the editing process. In order to make the video look slow, the computer interpolates the extra frames needed. So, to make one second of video shot at the traditional rate of 29.97 fps slowed down to 50% (now making that clip two seconds), the computer has to create 30 additional frames that did not originally exist. This interpolation can cause artifacts that will reduce the quality of video, or make it look jittery. However, if you have 60 frames per second to start with, then when you covert that footage to the traditional rate of 30 fps, the result is true slow mo that looks silky smooth. Get it? The 50p mode is good if you want 50% slow motion for a 24p video or a PAL video (which is 25p).

THE BIG Ps

So, what about 720p and 1080p. Is that video shot at 720 fps? Nope. When you see the big “p’s” (i.e. 720 and 1080 respectively), they’re referring to the vertical dimension of an HD video (standard definition video is 720 x 480, so at times you may see 480p). The two popular HD flavors are 1280 x 720 and 1920 x 1080 (which is full sized HD). 720p refers to a 1280 x 720 HD video shot in progressive mode, etc. (Note: oOn the Canon 7D, the 50p and 60p modes are only in 720p, not the full sized 1080p).  Some of the traditional video camcorders shoot in 1080i, which is (you guessed it) 1920 x 1080 HD that is interlaced. (Image below is from an Ecoustics.com article on full resolution HD.)

In essence, I wrote this whole article just to say: if the number before the “p” is less than 100, it’s referring to the frame rate. If it’s over 400, it’s referring to the HD dimensions, specifically the vertical dimension.

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