Cinematic Motion Blur – 180° Rule

Setting the right shutter speed is an important task in video and timelapse. Although setting it in a video is easier than in the timelapse, it’s not always a piece of cake. This text was created during the work on the bigger article about ND filters and motion blur in timelapse. However, it gets quite long, and while I consider it as an introduction for beginners, I decided to exclude it to another post (I still think I write too long articles, but I want it to be as complex as it could be).

Motion blur is very interesting topic. Once you understand the 180 degrees rule, check out how you can apply that in your films using ND filters: ND Filters in Filmmaking – Make Your Footage Cinematic 

So here you’ll learn the general rule, naming and the differences that gave us various shutter speeds. In the following article, you will find how to set desired shutter speed in hard conditions (especially when it’s too bright). Check out the post: Shutter speed in timelapse – ND filters. If you dont want to miss new tutorials, click the little red bell in the bottom left corner. You will get notification every time I release new exciting content (don’t worry, it’s not that often ;))

What is a motion blur and where does it come from?

The easiest way to explain that is to  compare it to the photography. In the video, each individual frame is created as a photo. As in photography, you can freeze or blur the movement in the frame (on each one individually). Of course, the blur depends on the shutter speed that you’ve selected.

I made a quick setup for this tutorial for a better explanation of how the shutter speed affects the image. In the picture below you can see a pinwheel mounted on a tripod, a fan that propels the pinwheel and of course a camera, white backcloth and a light source not included in the picture. The fan provides a constant speed of the pinwheel, and it’s very important for this test. I take a few pictures of the rotating pinwheel with different camera settings. Shutter speed is the important part here, I change the other settings only to maintain the same exposure.

In the picture below you can see the effect of this test – a visual representation of the motion blur. In the left one, you can’t even say what’s in the picture, and that’s the longest exposure time. On the other hand, the right one looks still, because the exposure time (shutter speed) is short enough to freeze the movement. Of course, the parameters placed in the image depend on the speed of the subject (and also the amount of light available). Sometimes 1/800 will freeze the movement, and sometimes you’ve got to go even further to do that.

Let’s talk about the second pinwheel, this one shot at 1/50s. As you can see, it’s far from being razor sharp. The shutter speed was set to 1/50s, that means the camera is capturing the light through that time. The pinwheel is rotating so fast, that through that 1/50s it rotates several degrees and it’s captured in the image. To be sure you understand – the camera starts taking a picture when the blades are, let’s say at 0° and finishes the exposure at for example 15°. That 15° rotation is captured in the picture and appears as a blur.

Naming

There are two different namings here – shutter speed and shutter angle. Shutter speed is a time of single frame exposure defined in seconds (just like in photography), for example 1/48s, 1/100s etc. Shutter angle is more complicated, as it gives us shutter speed that depends on fps (frames per second) that you’re shooting. I’ll explain that later in the text. Some cameras have only shutter angle setting, and in some you can choose between these two (even in small cameras, like GH4).

180° rule

The main rule in setting shutter speed in a video is the 180 degree rule. It tells, that you should set your shutter speed to twice of your frame rate (to be exact – 1/[2x fps]). So, according to this, if you’re shooting 24fps (frames per second) you should use a shutter speed of 1/48s, for 25 fps – 1/50s, for slow motion 200fps – 1/400s etc. Simple, right? 😉

When I first heard of that rule I’ve had two questions, which helped me understand it correctly (and then break it on purpose): what does it do to my image (how will it look like if you choose different shutter speed) and where does it come from.

Let’s start with the second one: the 180° rule comes from the old school film cameras. The gif shows how the shutter mechanisms work in a film camera. You can see rotating semicircle, that works as a shutter mechanism. And that’s where the degrees came from – the film is exposed during 180° rotation of the circle (the “hollow” semicircle). During the second 180° rotation the film is covered (so there’s no exposure) and it’s time to move the film to the next frame.

A film camera has got to have that time to physically move the film down for the next exposure, so there can’t be shutter angles higher than 180°. There could be lower – for example 90° (1/96s for 24fps), but it was used only in specific situations. The 180° shutter is considered to be more natural. In today’s digital cameras there isn’t any physical shutter (of course, in video mode, in photography there is a physical shutter, but different than in film cameras), so exposure is accomplished by electronics of the camera. The camera doesn’t need time to move the film, so you can achieve even 360° exposure (for example 1/25s for 25fps). Below, you can find the visual representation of different shutter angles (yellow is the hollow part).

And now it’s time for the first question: what does it do to the image?

The shutter speed determines the amount of motion blur in the video – on every frame (and of course the amount of light that reaches the sensor or film, but that’s not the point of this article). We, as timelapsers, know best, that the final video is a set of pictures. Shutter speed is the time of exposure for each frame, like in photography. So if you want to ‘freeze’ the movement in the frame you set high shutter speed (for example 1/200s, 1/500s etc), if you want to blur the movement you set longer shutter speed – 1/50, 1/25 etc.

However, unlike in photography, you don’t watch each frame separately here. At 1/50s each frame with moving objects can look bad and blurry, but when you play 25 frames like that in one second it’ll look surprisingly naturally. Furthermore, the video without motion blur can look odd to viewers. That’s because human eye also sees the motion blur, so video shot according to the 180° rule would look closer to what we all see. You can try to fast wave your hand in front of your eyes. It’s not crispy sharp, but blurry.

In the gif below you can see our pinwheel again. This time it’s a video shot at 25fps with three different shutter speeds: 1/25s – 360° shutter angle, 1/50 – 180° and 1/500s – about 20°. Here I can proof, what I’ve written above. The 1/50s shutter speed, so 180° shutter angle looks closer to what my eyes saw than any other shutter speed. You can take my word for it or just check it by yourself. You can simply build the same setup I did 😉

So, now you know the rule. Should you stick to it no matter what’s going on? Of course not! You should know the rules to break it on purpose (not by accident). There are a few examples of doing that in big films. In Saving Private Ryan (1998) cinematographer, Janusz Kaminski used a shutter angle of 45° for explosions and 90° for most of the running shots (which gave 1/198s and 1/96s).

Often, when you see an explosion with a 180-degree shutter it can be a thing of beauty, but a 45-degree shutter looks very frightening. [Steven Spielberg]

I hope now you know why setting your camera to auto settings is like letting someone else decide how your films gonna look like.

In conclusion – using 180° rule will give you image that’s close to what people see and what we all have watched in cinema for years. Like every rule, you can break it to get the look that you want.

Thanks for visiting my blog, I hope you’re not disappointed 😉 Now, when you’re a specialist in motion blur rules, read my article about ND filters and motion blur in timelapse! It’ll be available soon on the blog.

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