The 180-degree rule

Learning goal

When shooting video, you will be able to choose the shutter speed not to freeze movement as in photography, but to give a natural-looking motion blur that suits the frame rate. You will explain the 180-degree rule, which suggests keeping the shutter speed close to the inverse of twice the frame rate, and the concept of shutter angle it takes its name from, and calculate the corresponding shutter speeds for common frame rates. You will tell why a very short shutter speed shows movement choppily and why a very long one leaves too much blur. You will know how to reduce the excess light with an ND filter and stop math when the shutter speed stays fixed in daytime, and learn how to choose the shutter speed so that lamps running on mains electricity don't create flicker in video. With the shutter speed fixed, you will think about how to expose a face that lacks light by adding light rather than raising the ISO.

The question

One shot of the bakery video shows the baker's hands kneading the dough. As someone who has spent years freezing movement, your friend from the club has left the shutter at one thousandth of a second. Pausing and looking at a single frame, the hands are flawless: every finger sharp, the grains of flour frozen in the air. But on playback something strange appears: the hands move not by flowing but in small jumps, as if teleported a few millimeters forward at every instant. The image tires the eye.

Someone at the club shoots the same shot again with the shutter speed at one fiftieth of a second. In a single frame the hands are slightly blurred, the edges of the fingers soft. But on playback the movement suddenly becomes natural: the hands glide smoothly over the dough. Why should the shutter speed in video be twice the frame rate? How can a setting that looks worse in a single frame be better in a flowing image?

Context

In the shutter speed lesson you defined shutter speed as the time the sensor stays open to light, and saw that a short shutter speed freezes movement (Lesson 8.4). In the timelapse lesson, too, we discussed that when frames are played one after another, a short shutter speed makes clouds advance in jumps while a long shutter speed softens the flow (Lesson 22.6). This lesson applies the same principle to video shot at normal speed.

In photography, choosing a shutter speed is usually about freezing movement or blurring it deliberately. In video, on the other hand, each frame is replaced by the next a moment later, and the eye sees the frames not one by one but as a flow. For this flow to look natural, movement needs to leave a little trace in each frame.

Simple explanation

What does the rule say?

The 180-degree rule is the habit that suggests keeping the shutter speed in video close to the inverse of twice the frame rate, that is, close to half of each frame's duration. In a video shot at twenty-five frames per second, each frame lasts one twenty-fifth of a second; half of this duration is one fiftieth of a second. At thirty frames per second the shutter speed becomes one sixtieth of a second, at fifty frames one hundredth, at sixty frames one hundred twentieth. At twenty-four frames per second the exact value is one forty-eighth of a second; on most cameras the nearest value, one fiftieth, is chosen.

Shutter angle is a measure that describes, as the degrees of a circle, for how much of a frame's duration the sensor stays open to light. The rule's name comes from the rotating, half-circle shutter of old film cameras: the shutter turns one full rotation for each frame and its open half, that is, its 180 degrees, lets light fall on the film. Because a full rotation is 360 degrees, 180 degrees means half of the frame's duration. The shutter time is found by dividing the ratio of the shutter angle to 360 by the frame rate: with 180 degrees at twenty-five frames per second, this is one fiftieth of a second.

This rule isn't a law, but a starting point the viewer's eye is used to. Deliberately choosing a different shutter angle can be used to add tension or a dreamlike softness to a scene; but doing this as a choice is different from doing it out of habit.

Why do short and long shutter speeds look different?

Staccato motion is movement freezing completely in each frame of a video shot with a very short shutter speed, and advancing in small jumps instead of flowing when the frames are played one after another. At one thousandth of a second the baker's hand stops at a sharp point in each frame; in the next frame it stops sharp again a few millimeters further on. There is no trace of the path in between, and the eye perceives this gap as a jump. Because the eye sees movement in the real world as a flow, this image feels strange and tiring.

When the shutter speed is close to half of the frame duration, movement leaves a slight trace in each frame. This small blur connects the end of one frame to the start of the next, and the eye sees the movement as unbroken. When the shutter speed is too long, on the other hand, for example at one twenty-fifth of a second, each frame stays open for its whole duration and movement blurs too much; fast movements look misty and scattered.

What is done in daytime and under lamps?

When the shutter speed stays fixed in video because of the rule, what the photographer has left to adjust the light is the aperture and the ISO. Shooting outdoors in daytime at one fiftieth of a second often means far too much light. According to the sunny 16 rule, in sunlight the pair ISO 100 and f/16 calls for an exposure of one hundredth of a second; opening the aperture to f/8 for depth of field adds two stops of light and the correct shutter speed rises to one four-hundredth of a second. To get down to one fiftieth of a second you need to cut three stops of light; a three-stop ND filter does this. Getting down to one fiftieth from the habit of one thousandth of a second brought from photography, on the other hand, is about four and a third stops.

A variable ND filter is a filter that reduces light steplessly by rotating two polarizing glasses relative to each other. As the light changes during the shoot, it lets you adjust the exposure by turning the filter, without touching the shutter speed. At very dark settings a dark cross pattern can form in the image; that is why you need to be careful at the filter's darkest end.

Mains frequency is the value showing how many times per second the electricity in the socket changes direction, and depending on the region it is fifty or sixty hertz. Many lamps, on this electricity, brighten and dim twice the mains frequency per second; on a fifty-hertz grid this is a hundred pulses per second. When the shutter speed is an exact multiple of the pulse duration, each frame gathers the same number of pulses and the image stays steady. One fiftieth or one hundredth of a second meets this condition on a fifty-hertz grid; one sixtieth of a second, on the other hand, can gather a different amount of light in each frame, and the light flicker you saw in the shutter speed lesson shows in video as fluctuating brightness and bands.

When light is low with the shutter speed fixed, the easiest way is to raise the ISO; but this increases noise in the image, and it shows most in the dark areas. In the bakery's dim corner, as the ISO rises, the dark-skinned baker's face fills with noise on its shadow side and the warm tone of the skin turns into a muddy gray. A better way is to add light: moving the baker closer to the window, or holding a light-colored card on the dark side of the face, keeps the ISO low and leaves the skin clean. Outdoors the opposite happens: in the sun, a light-skinned forehead easily clips without an ND filter.

Visual evidence

You set up the dough shot again. You set the camera to twenty-five frames per second and the shutter speed to one fiftieth of a second. In the bakery's dim corner you see the ISO rise and noise appear on the shadow side of the baker's face; when you move them a step closer to the window and hold a white card on the shadow side, the ISO drops and the face cleans up.

On playback the hands glide smoothly over the dough. The fluorescent lamps on the ceiling are steady; when you try one sixtieth of a second the image fluctuates slightly, and when you go back to one fiftieth it settles. In the shot you film outdoors in front of the bakery, on the other hand, there is too much sun: at f/8 you attach a three-stop ND for one fiftieth of a second, and the baker's face stays in its own tone.

Watching it side by side with the first day's shot, the difference is understood at once. In the thousandth-of-a-second shot the hands jump to the next position at every instant and the eye struggles to follow this choppiness; in the fiftieth-of-a-second shot the same hands glide over the dough. In the first attempt in the dim corner the shadow side of the face was grainy and gray; in the shot with added light, the same face is smooth and warm.

Common mistakes

Summary

The 180-degree rule gives a natural motion blur by keeping the shutter speed close to half of each frame's duration, that is, the inverse of twice the frame rate; shutter angle, which takes its name from the half-circle shutter of old film cameras, describes this in degrees, and a very short shutter speed produces staccato movement while a very long one produces too much blur. With the shutter speed fixed, excess daylight is reduced with an ND or variable ND filter, a shutter speed matched to the mains frequency prevents lamps from flickering, and when light is low, adding light instead of raising the ISO protects especially the shadow side of dark-skinned faces from noise.

Mini assignment

In this assignment you will shoot the same movement as video with different shutter speeds and compare them.

1. Write the shutter speeds that fit the 180-degree rule for twenty-four, twenty-five, thirty, fifty and sixty frames per second.
2. Shoot the same movement, waving a hand or pouring water, at twenty-five frames per second once at one thousandth of a second, once at one fiftieth and once at one twenty-fifth, and compare the three by playing them back.
3. On a sunny day, calculate how many stops of ND are needed to get down to one fiftieth of a second with the aperture and ISO you chose.
4. Under fluorescent or LED light, try one fiftieth and one sixtieth of a second; if there is a person in the frame, note how their face looks when the ISO rises in dim light, and shoot again adding light.

Check yourself

• Given a frame rate, can you calculate the shutter speed that fits the 180-degree rule?
• Can you tell apart staccato movement and natural motion blur with your eye?
• When light was low, did you expose the face by adding light instead of raising the ISO?

Glossary

Mini quiz

1. In a video shot at twenty-five frames per second, which shutter speed fits the 180-degree rule?

• A) One twenty-fifth of a second
• B) One fiftieth of a second
• C) One thousandth of a second
• D) One hundredth of a second

2. Why do the hands move in jumps in video shot at one thousandth of a second?

• A) Because the frame rate is too high
• B) Because the white balance is wrong
• C) Because image stabilization is off
• D) Because movement freezes completely in each frame and no trace of the path in between remains

3. In the sun at f/8 and ISO 100, the correct shutter speed is one four-hundredth of a second. How many stops of ND are needed to get down to one fiftieth of a second?

• A) Three stops
• B) One stop
• C) Six stops
• D) Nine stops

4. Under fluorescent light on a fifty-hertz grid, which shutter speed is most suitable for preventing flicker?

• A) One sixtieth of a second
• B) One hundred twentieth of a second
• C) One fiftieth of a second
• D) One thirtieth of a second

Answers

1. B — Each frame lasts one twenty-fifth of a second; half of this is one fiftieth of a second.
2. D — A short shutter speed freezes movement in each frame; the eye perceives the gap in between as a jump.
3. A — From one four-hundredth to one fiftieth, each step doubles: one two-hundredth, one hundredth, one fiftieth; three stops in total.
4. C — One fiftieth of a second gathers an exact multiple of the hundred light pulses per second, and each frame gets the same light.

Summary card

• Shutter speed ≈ 1 / (2 × frame rate): 25 → 1/50, 30 → 1/60, 50 → 1/100.
• A very short shutter speed gives choppy movement, a very long one misty movement.
• In daytime, reduce light with an ND or variable ND.
• On a fifty-hertz grid, 1/50 or 1/100 prevents flicker.
• When light is low, add light instead of ISO; protect the shadow side of the face from noise.

Bridge to the next lesson

The dough shot flowed smoothly now. This time your friend wanted to show the flour the baker tossed into the air slowed down. When they reduced the shot filmed at twenty-five frames per second to half speed in the editing program, the flour cloud didn't drift softly but advanced haltingly, skipping frame by frame. In the camera's menu, next to twenty-five there were other values too, such as fifty, sixty and a hundred twenty. 24, 30, 60, 120: what is each one for? The next lesson tells about how frame rate and resolution are chosen for cinematic, standard and slow-motion footage.