Understanding Slow Motion in Video

What is Frame Rate?

Frame rate is the speed at which images are shown, and it’s usually expressed as “frames per second,” or FPS. Each image represents a frame, so if a video is captured and played back at 24fps, that means each second of video shows 24 distinct still images. The speed at which they’re shown tricks your brain into perceiving smooth motion.

Frame rate greatly impacts the style and viewing experience of a video. Different frame rates yield different viewing experiences, and choosing a frame rate often means choosing between things such as how realistic you want your video to look, or whether or not you plan to use techniques such as slow motion or motion blur effects. For example, movies are usually displayed at 24fps, since this frame rate is similar to how we see the world, and creates a very cinematic look.

How Our Eyes are Affected

The human eye is capable of differentiating between 10 and 12 still images per second before it starts just seeing it as motion. That is, at an FPS of 12 or less, your brain can tell that its just a bunch of still images in rapid succession, not a seamless animation. Once the frame rate gets up to around 18 to 26 FPS, the motion effect actually takes effect and your brain is fooled into thinking that these individual images are actually a moving scene.

So if a frame rate is too slow, motion looks jagged, but if it’s too fast you can have problems too. Live-action movies filmed at 48 FPS tend to have that certain soap-opera effect people hated in The Hobbit. That’s because one major component of making the motion seem real and lifelike is motion blur. In the natural world motion blur is simply the loss of detail you get when you’re looking at something that’s moving fast, or when your eyes are moving fast as they look at something.

Understanding Slow Motion

There are two ways to capture slow-motion video. The first is within the camera at the time of shooting, and the second is during post production. If you want the shot to be in slow motion from the beginning, however, it is best to do it in camera. This leads to the easiest post production workflow and tends to give better quality in the long term.

The standard speed for cinematic pictures (in the U.S.) is about 24 frames per second. This creates a fluid movement for each frame that is the most realistic. That may seem logical, but the reality is that you can actually shoot at a higher frame rate than what things are being played back as. So let’s say you shot something at 48 frames per second, but want to play it back at 24 frames per second. This means you are watching it at half the speed that you shot it.

And there you have it – slow motion. That’s how it works. But there’s more to it.

Shutter Speed in Slow Motion

A big part of shooting moving pictures is the shutter speed. If you know anything about cameras, you probably know the shutter opens for only for a fraction of a second as to not let in too much light or capture too much movement, which causes motion blur. That said… moving pictures actually need motion blur to create the illusion of movement. If everything was too sharp and pristine, it would look very artificial and choppy.

In the contrary, when we are shooting in slow motion, it’s necessary to utilize less motion blur since the images are staying on the screen for so long. Then it’s very noticeable!

The general rule of frame rates is that the denominator of your shutter speed (i.e. 1/48, 1/96, 1/144) should typically be double your frame rate (or close to it). Fortunately, this is very easy to do if you are shooting on a DSLR or high-end video camera!

This means you if you shoot at 24 frames per second, your shutter speed should be 1/48 (or 1/50 if your camera is unable to do 1/48). While shooting slow-motion video at 48 frames per second, it should be 1/96. Easy enough.

 

There are ways to simulate slow-motion, but realistically, none of them are that great. For one, you could take something you shot in 24 frames per second, put it in your editing timeline, and then try to stretch it out to be oh… say, 50% the speed of the original. Guess what? It will work, your editing software will do it. The problem is this: frames will be reused, and basically, you’ll just be watching frames at half their normal speed, destroying the illusion of movement.

Choosing a frame rate requires some thought and if you take into consideration the points lined out above you should find success. If you’d like to play around with frame rates and see a little more about how they work, this site offers some fun ways to experiment.

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