Frame rate and resolution

Learning goal

Before starting a video shoot, you will be able to decide which frame rate and which resolution to choose by looking at how the video will be used. You will tell apart the purposes for which twenty-four, twenty-five, thirty, fifty, sixty and a hundred twenty frames per second are chosen, and learn that showing a movement in slow motion requires shooting it at a higher frame rate than it will be played at, and how to calculate the slow-motion factor. You will calculate in stops why, at a high frame rate, the 180 degree rule asks for a shorter shutter speed and therefore more light. You will calculate the pixel difference between Full HD and 4K, and know why some cameras can crop the image at a high frame rate or a high resolution. Keeping in mind how slow motion stretches and exaggerates a facial expression, you will show a person their slow-motion footage before using it.

The question

For the bakery video, your friend wants to show the flour the baker tosses onto the counter in a slowed-down shot. They shoot the shot at twenty-five frames per second and halve the speed in the editing program. Instead of drifting softly, the flour cloud advances haltingly; each frame is shown twice and the movement looks choppy.

Digging through the menu, they see that the list of frame rates extends up to a hundred twenty; and resolution options such as Full HD and 4K. With some settings they also notice the image narrowing a little. 24, 30, 60, 120: what is each one for? Which setting do you need to choose to really slow down the flour?

Context

In the timelapse lesson we defined frame rate and compressed time by playing frames at normal speed (Lesson 22.6). In the 180 degree rule lesson, too, you saw that the shutter speed depends on the frame rate, and that at twenty-five frames per second one fiftieth of a second is chosen (Lesson 24.2). Slow motion is the opposite of timelapse: instead of compressing time, it stretches it.

Frame rate and resolution decide how a video will look and where it can be used. But each has a price: a high frame rate asks for more light, a high resolution produces bigger files, and on some cameras the two can't be chosen at the same time.

Simple explanation

Which frame rate is chosen for what?

Twenty-four frames per second gives a sense of movement the viewer associates with cinema, coming from feature films having been shot at this speed for a long time. Twenty-five and thirty frames per second are the standard values for everyday videos. Which is chosen often depends on the region: in regions whose grid is fifty hertz, twenty-five and fifty are common, and in regions with sixty hertz, thirty and sixty. This choice is also related to the lamp flicker in the previous lesson; on a fifty-hertz grid, twenty-four frames per second carries a risk of flicker under some older types of lamp.

Project frame rate is the frame rate at which a video will be played in the editing program and delivered. All the shots in a project are played at this speed. Setting the project frame rate before the shoot makes sure all the shots are compatible with each other; adding a thirty-frame shot to a twenty-five-frame project can make the movement stutter slightly.

Fifty and sixty frames per second show fast movements more smoothly and allow slowing down by two times. A hundred twenty frames per second, on the other hand, is for slow motion: it is used to show a movement in more detail than the eye can catch.

How does slow motion form?

Slow motion is slowing down a movement by shooting it at a higher frame rate than it will be played at and playing it at the project frame rate. The slow-motion factor is the number found by dividing the shooting frame rate by the project frame rate. When a shot filmed at a hundred twenty frames per second is played in a project of twenty-five frames per second, it slows down about 4.8 times; a toss of flour that really lasts one second lasts about 4.8 seconds on screen. A shot filmed at fifty frames per second slows down two times in the same project.

Slowing down in editing a shot filmed at twenty-five frames, on the other hand, adds no new information; the program fills the gap by showing each frame several times or by blending frames into each other. That is why the flour cloud advances haltingly. True slow motion requires recording the frames during the shoot.

A high frame rate has a price. According to the 180 degree rule, at a hundred twenty frames per second the shutter speed becomes one two-hundred-fortieth of a second, in practice one two-hundred-fiftieth. Going down from one fiftieth of a second to one two-hundred-fiftieth means an exposure five times shorter, that is, about two and a third stops less light. Indoors, this shortage of light often forces you to raise the ISO or add light. Some LED lamps can also create fluctuating bands in the image at a high frame rate; a short test before the shoot shows this.

Slow motion also stretches a face. A blink, a face scrunching up as it gets covered in flour, lips tightening last for seconds in slow motion and look much more pronounced than they are, sometimes comical. Showing a person their slow-motion footage before publishing it gives them a say in how they look.

What does resolution change?

A Full HD video is 1920 pixels wide and 1080 pixels high; this makes about 2.1 million pixels in each frame. 4K UHD is a video resolution 3840 pixels wide and 2160 pixels high, with about 8.3 million pixels in each frame; four times Full HD. 4K allows you to crop and zoom the image in editing or keep sharpness on a large screen, but files grow and the computer struggles more.

Video crop is some cameras using only the middle of the sensor at certain resolution or frame rate settings, and the image being recorded at a narrower angle than in photography. This makes a shot filmed with the same lens look closer than expected. Which settings crop varies from camera to camera; checking this in the menu or in the frame on the screen keeps a wide shot from narrowing unexpectedly.

Visual evidence

You set the project frame rate to twenty-five frames per second, and this time shoot the flour shot at a hundred twenty frames per second. The shutter speed drops to one two-hundred-fiftieth of a second; the bakery's dim corner isn't enough, so you open the window and move the baker closer to the light.

When you play the shot at project speed in the editing program, the flour cloud spreads over the counter about 4.8 times slower, smoothly. In the version of the same toss filmed at twenty-five frames and halved, the cloud advanced haltingly; at a hundred twenty frames, on the other hand, every grain drifts on its own path.

When you show the baker the shot, they laugh at a moment lasting several seconds in which they squeeze their eyes shut and their face scrunches up as the flour touches it, but they don't want that part to be in the video. You cut the shot before the flour reaches their face. You notice the image narrowing a little at the camera's hundred-twenty-frame setting; for a wide counter shot, you go back to twenty-five frames in Full HD.

Common mistakes

Summary

Twenty-four frames per second gives a feeling particular to cinema, twenty-five and thirty the everyday standard, fifty and sixty smooth movement, a hundred twenty slow motion; all shots are played at the project frame rate set in advance, and slow motion slows down by the slow-motion factor when a shot filmed at a high frame rate is played at this speed. Because of the 180 degree rule a high frame rate asks for a shorter shutter speed and more light, 4K UHD carries four times the pixels of Full HD, and at some settings video crop narrows the image; because slow motion stretches and exaggerates a face's expression, the footage is shown to the person before being used.

Mini assignment

In this assignment you will record a single movement at several frame rates and compare the results by slowing them down in editing.

1. Write the frame rates and resolutions your camera offers; check from the frame on the screen at which settings the image is cropped.
2. For a project of twenty-five frames per second, calculate the slow-motion factor of shots filmed at fifty and a hundred twenty frames, and their shutter speeds according to the 180 degree rule.
3. Shoot the same movement, pouring water or throwing a ball, once at twenty-five and once at fifty or a hundred twenty frames; slow down the twenty-five-frame shot in editing and compare it with the high-frame-rate shot.
4. Write in stops how much light you lose at the high frame rate. If you filmed a person in slow motion, show them the footage and ask which moments they don't want used.

Check yourself

• Can you choose the frame rate for a shot according to how the video will be used?
• Can you calculate the slow-motion factor and the light loss at a high frame rate?
• Did you show the person how a face looks in slow motion and get their approval?

Glossary

Mini quiz

1. If a shot filmed at a hundred twenty frames per second is played in a project of twenty-five frames per second, about how many times does it slow down?

• A) Two times
• B) Twenty-five times
• C) About 4.8 times
• D) It doesn't slow down at all

2. Why does the flour cloud filmed at twenty-five frames and reduced to half speed in editing advance haltingly?

• A) Because no new frame is added, the program shows the same frames again
• B) Because the resolution is low
• C) Because the shutter speed is too long
• D) Because the color profile isn't suitable

3. About how much loss of light does going down from one fiftieth of a second to one two-hundred-fiftieth mean?

• A) Half a stop
• B) Five stops
• C) One stop
• D) About two and a third stops

4. For the slow-motion footage in which the baker squeezes their eyes shut as the flour touches their face, which is the most suitable behavior?

• A) Using the footage without telling them
• B) Showing the footage to the baker and asking which moments they don't want used
• C) Slowing the footage down even more
• D) Turning the footage into black and white

Answers

1. C — A hundred twenty divided by twenty-five makes about 4.8; a one-second movement lasts 4.8 seconds.
2. A — Slowing down in editing adds no new information; the program fills the gap by repeating the same frames.
3. D — An exposure five times shorter is, by the base-two logarithm, about two and a third stops.
4. B — Slow motion stretches and exaggerates expression; the person should have a say about their own footage.

Summary card

• 24 cinematic, 25/30 standard, 50/60 smooth, 120 slow motion.
• Set the project frame rate from the start.
• Slow-motion factor = shooting frame rate / project frame rate.
• A high frame rate asks for a shorter shutter speed and more light.
• 4K = Full HD × 4; check the crop setting; show the slow-motion face to the person.

Bridge to the next lesson

The slow-motion flour shot charmed the baker's family. But when the video came to the part where the baker describes the bakery, the family started looking at each other. The baker's voice echoed off the bare walls, the hum of the oven slipped in between the sentences; someone asked, "what are they saying?" A member at the club said that viewers often put up with a blurred image, but close the video at sound they can't understand. Does the audience forgive bad picture or bad sound? The next lesson tells about the basics of recording sound with an external microphone and a quiet space.