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Author Topic: March 2009: Video Frame Rates (24p, 25p, 30p, 60i)  (Read 56975 times)
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« on: May 27, 2009, 03:36:57 PM »

Video Frame Rates (24p, 25p, 30p, 60i)


Both camcorders and some video capable digital cameras offer not only the ability to select resolution such as 640x480, 1280x720, or 1920x1080 but also offer the ability to select from one of a number of frame rates such as 24, 25, 30 or even 60 frames or "fields" per second.  Do you know which one to use?  Have you heard the statement that 24p "looks like film"?  Does it really?  And what exactly does "looks like film" even mean?  Let's take a look. 


The i's and the p's

Simply stated, the "i" or "p" after the capture rate indicates whether or not the video is interlaced or progressive.  60i for example indicates that the video will be 60 "fields" per second where each field is only half of a frame.  The word "interlaced" means that each field consists of (typically) the odd rows or even rows in the picture.  Since each update (very 1/60th of a second) only updates half the frame, you never actually see an entire frame.  What you are actually seeing is half a picture mixed with the other half taken 1/60th of a second later.  Of course, this happens so fast that our eyes perceive the data as if it were full frames.  In contrast, 30p video also displays one entire frame every 1/30th of a second so an entire frame is displayed at once.

Here's the catch and why 30p is generally regarded as better than 60i as far as resolution.  Because 30p video displays a full frame at a time, an entire frame or snapshot of the scene is displayed for 1/30th of a second and then the next frame is displayed for 1/30 of a second and so on.  If you could freeze time at any point in the video and look at your screen, you'd see a proper (full and unbroken) photo of the scene.  With 60i, only half of the entire picture is updated every 1/60 of a second which means at any given time, only half the frame is displayed and the other half is 1/60th of a second "old".  This can cause artifacts and can tend to blur detail of moving objects because mixing half of one frame with half of a different frame is obviously not optimal.  Such is the case with any frame rate that is followed by the letter "i" to indicate interlacing.  On a positive note, 60i can give the impression of less flicker when viewing fast moving objects such as sports in which case the 60i may look smoother even if slightly lower resolution.



If you search the web for 24p, you'll find a lot of claims that "24p just looks more like film".  What people are referring to here is the look of film (motion pictures) versus video (HD video shot for TV for example).  What's the difference?  For me, 99% of the difference relates to depth of field which has nothing to do with frame rate.  Motion pictures are normally shot with film cameras that have a large capture area which allows film to have a shallow depth of field where the subject is sharp and the background is blurry.  This shallow depth of field tends to spotlight the subject and the blurring of the background often removes distractions.

In contrast to film, video is often shot with smaller cameras, many of which have small digital sensors.  The smaller sensors usually equate to a greater depth of field which causes most video sources to show both the subject and the background as being in focus at the same time.  When you are watching something like a documentary or reality show shot in HD, notice how most of the time the subject and background are both in focus at the same time even in macro shots!  The ability to create a soft/blurry background is what gives movies their cinematic feel and having both the subject and the background in focus makes the content look more like video and less like film.

So how does this all relate to 24p?  It doesn't and that's my point: 24p has a reputation for looking more like film when in fact the frame rate has little to do with what separates film and video.  The only difference between 24p and 30p in video is that 24p will actually show slightly more "strobing" effect, most visible when the camera is panning.  Film makers are aware of this and often avoid panning high contrast scenes where strobing might occur.  If you use 24p in your own video and you are not careful about panning, you'll be more prone to the strobing effect than you will at 30p.  So really the only way 24p can "look more like film" than something like 30p is if you are able to see strobing during certain scenes: something I don't particularly think is a positive aspect of film!

In my opinion, 24p is more useful for video (DVD and Blu Ray) content that has been converted from film.  Almost all motion pictures are shot at 24 frames per second so the ability to convert to the digital media (DVD or Blu Ray) and retain the original 24 fps is a benefit.  If you are starting out with digital to begin with as you would with a camcorder or digital camera, however, 30p makes more sense (or 25p if you are in non-U.S. contries that use 50hz power).  Your videos will likely be destined for either monitors or televisions that run at 50 or 60 hz which are multiples of 25 or 30, so no need for 24 fps. The biggest thing 24p has going for it is the possibility of a savings in storage space and better potential for portability: 24p is often regarded as the best format to use if you produce something that is to be distributed internationally.  For most of us with digital cameras and camcorders, we'll probably be using the video mostly in our own region of the world so we don't have to worry as much about portability and compatibility in other areas of the world.



If you are anywhere but the U.S. or Canada, your power grid likely runs at 50hz and your video equipment and resulting videos may be better at 25p.   Due to the fact that 30 fps (NTSC) video is rather easily converted to 25 fps (PAL) by most equipment, some cameras and camcorders only offer a 30p or 60i video format.  Here in the U.S., 25p isn't particularly useful because our power grid runs at 60hz so most electronic equipment in the U.S. tends to favor a multiple of 30 or 60.  Fact is, most of the rest of the world (outside the U.S., Canada, Brazil, and a few other countries) use 50hz PAL for video rather than 60hz NTSC so in those locations, 25p video may be optimal as long as you don't intend to produce something that is likely to be viewed internationally.



For people in the U.S., Canada, and countries that follow the 60hz NTSC video standard, 30p may be your best bet.  It's a faster frame rate than 24p and your HDTV or computer monitor likely refreshes at 60hz making the 30p frame rate a nice match for your equipment.  Again, if you are concerned with portability and compatibility with other countries that use the 50hz PAL video standard, 24p is generally considered the most internationally accepted format yet the reasons for this portability are often weak: 24p is often preferred simply because it matches neither of the world power/video standards of 50/60hz and therefore doesn't make your preference for one region or the other obvious.  That said, I've not heard of any issues with PAL equipment (25hz) displaying an NTSC based (30hz) video.  Simply put, it is easier to go from 30p to 25p than it is 24p to 25p without trying to run the video too fast (running 24p at 25p speeds) and hoping that no one notices.  Going back and forth between 30p and 25p can be done with a combination of 3:2 pulldown methods without affecting video playback speeds.



As mentioned earlier in this article, 60i video basically amounts to 60 fields per second because you are not capturing entire frames.  Simply put, 60i amounts to capturing 60 half frames per second.  At first glance, it may sound like 60i and 30p are equivalent but they are not.  30p video generally results in better resolution and sharpness than 60i video while 60i video excels in fast action video such as sports.  Since 60i video updates half the frame each cycle, resolution is reduced due to the fact that an entire frame is never captured at once.  Since at least part of the image is updating every 1/60th of a second, however, flicker/strobing is reduced and 60i can look smoother to the eye even though overall, the data update rate is the same as 30p.  Simply put, 60i can be useful if you plan to video a sporting event or if you are taking video where you must do a lot of panning from side to side.  30p is generally better for most slow to average speed pans and motion due to the ability of 30p to capture more detailed information and sharpness.



In this article, I wanted to cover various video frame rates to give readers a general idea of the pros and cons of each.  I see that many people are confused about frame rates and many times people pick a particular frame rate for the wrong reason.  Hopefully this article will help in that regard without getting too technical.  I intentionally didn't discuss the fact that 30p is really 29.97 fps and that 24p is really 23.98 fps.  This article was written to assist the consumer/prosumer videographer with popular video formats and their purpose without getting lost in the nuts and bolts.  Best of luck with whatever video format(s) you choose!


Mike Chaney

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