Frame Rate

Nowadays, people carry powerful video cameras around with them everywhere they go.  Cell phones make it easy to capture video footage, and sites like YouTube make it possible for anyone to showcase their creativity behind the camera.  But just as holding a paintbrush won't automatically turn you into the next Picasso, there's a pretty large gap between owning a camera and becoming an awesome filmmaker.

For starters, what exactly is a movie, anyway?  A video camera is, first and foremost, a camera.  It does its job by taking a bunch of pictures in a short amount of time: when we watch those pictures played back, our brains fill in the gaps and create the illusion of motion.1

But we can be more precise than this.  Many video cameras (including the one inside of iPhones) film at a rate of 30 frames per second.  In other words, they take thirty pictures every second.  In other other words, the time between each picture is 1 ÷ 30 ≈ 0.033 seconds.  This doesn't mean it takes 0.033 seconds to snap a picture; rather, the camera takes a picture, takes another one 0.033 seconds after it started taking the first picture, and repeats.

Thirty frames per second is a common frame rate, but it isn't the only one.  For example, some GoPro cameras can film at rates as high as 240 frames per second, and as low as one frame per minute.  As you can imagine, shooting at 240 frames per second leads to some ultra smooth-looking video.  This means the camera is snapping a shot approximately every 1 ÷ 240 ≈ 0.004 seconds, meaning our brains have much less to "fill in."  On the other end of the spectrum, shooting at one frame per minute will result in some really jerky motion, since objects can travel pretty far in a minute.

Of course, this is assuming that we play back our video using the same frame rate it was recorded with.  But this isn't always the case.  In fact, many video players in the United States use a standard playback rate of 30 frames per second, regardless of how the video footage was originally created.

This means that what we watch on video may look different than it did when it was recorded.  For example, suppose a video is recorded at a frame rate of 240 frames per second (fps).  This means that if we record one second of footage, the camera will take 240 pictures.  But if we play this footage back on a video player at a rate of 30 frames per second, what took a single second in real life will take 240 ÷ 30 = 8 seconds to play back!  In other words, recording something at a high frame rate and playing it back at a lower frame rate will create a slow motion effect: events will take longer to watch played back than they took to record.  Here's an example of the effect in action.

On the other hand, what if you set up a camera to take pictures at a slow frame rate, like one frame per minute?  At that rate, it will take half an hour to capture 30 frames; that is, every second of video you want in your film will require 30 minutes of taking pictures.  This is why stop-motion animation takes so long to make, and it's also how time-lapse videos, like the one below, are created:

Of course, great movie-making requires harmony between a lot of components.  But with just a little frame rate know-how, you can create some truly mind-bending videos.  For example, who knew how much fun it could be to watch eggs break in slow-motion?

Teachers: interested in blowing your students' minds with movie magic?  Then check out our new lesson, Frame Rate!

1.  If you want to impress people at a dinner party, there's a name for the way the brain perceives motion between objects viewed rapidly in succession: the phi phenomenon.  You can thank it for any enjoyment you get out of watching movies.

Leave a Reply

Your email address will not be published. Required fields are marked *