Film set glossary — free, nothing uploaded

Q: What is the difference between a gaffer and a key grip?

The gaffer is responsible for everything that emits light — lamps, power, dimming. The key grip is responsible for everything that shapes or supports it: flags, diffusion frames, stands, dollies, rigging. One adds light, the other takes it away and holds it in place. On a large set they lead two separate departments and work as a pair.

Every trade builds a shorthand, and a film set builds more than most: half of it is a century old, and none of it is explained to newcomers. What follows is the vocabulary you will actually hear on a shooting day, defined by what each term changes rather than by what it translates to.

The crew

Job titles on a set describe responsibilities, not seniority. Knowing who to ask saves more time on a shooting day than any piece of equipment.

Gaffer: The chief lighting technician. The director of photography says what the image should look like; the gaffer decides which lamps, which power, which position get there. On a small shoot they are also the one who knows whether the building's electrics will hold.
Key grip: In charge of everything that shapes or supports — flags, stands, dollies, rigging — as opposed to everything that emits light, which is the gaffer's. The two work as a pair: one adds light, the other takes it away.
Best boy: The second in command of the electrical or grip department, and nothing to do with age or gender. Runs the crew and the equipment list so the gaffer or key grip can stay on set.
1st AD: First assistant director: runs the floor and the clock. They call the roll, keep the day on schedule and protect the director's attention. When a shoot finishes on time, it is usually them.
1st AC: First assistant camera, the focus puller. Holds focus by measurement and anticipation, not by looking at the monitor. On a fast lens wide open, the whole shot depends on this one person.
DIT: Digital imaging technician. Offloads the cards, verifies the copies, manages LUTs and color on set. The person who can tell you, before you wrap, whether the day is actually in the can.
Script supervisor: Keeps continuity and takes notes: which take, which lens, what the actor did with the glass in their left hand. The edit is assembled from these notes long after everyone has forgotten.
Boom operator: Holds the microphone as close to the actor as the frame allows, and moves it between speakers without entering shot or casting a shadow. The best sound on a set is a placement problem, not a microphone problem.

Light

Almost everything in lighting is a decision about direction, quality and contrast. The lamp matters far less than where it is and what is in front of it.

Key light: The dominant source, the one that decides where the shadows fall. Its position sets the whole mood before any other lamp is switched on. Move it and you have relit the scene; add lamps around it and you have only adjusted it.
Fill: Light that opens the shadow side, lowering contrast. Usually a bounce rather than a lamp, because fill should not create a second set of shadows.
Back light: Placed behind the subject, separating them from the background with a rim on hair and shoulders. It is the cheapest depth you can buy, and its absence is what makes an image look flat and amateur.
Negative fill: Black cloth placed to remove light rather than add it. On an overcast day or in a white room, taking light away from one side is often the only way to get any modeling at all. The most underrated tool on a set.
Practical: A lamp visible in shot that is part of the scene — a table lamp, a neon sign, a phone screen. It justifies the light in the frame and gives the audience a reason to believe the rest.
Diffusion: Anything that enlarges a source: silk, frost, a bounce board. A big source relative to the subject gives soft, gradual shadows; a small one gives hard edges. Softness is a matter of apparent size, never of power.
Flag: An opaque panel that stops light going somewhere — off a wall, out of the lens, off a forehead. Lighting is as much subtraction as addition, and the flag is how you subtract precisely.
Color temperature: Measured in kelvin: 3200 K for tungsten, around 5600 K for daylight. Mixing the two in one frame is a choice, not a mistake — but it has to be a choice, because the camera will not resolve it for you.
CRI: Color rendering index, from 0 to 100: how faithfully a source renders color compared to daylight. Cheap LEDs can measure the right kelvin and still render skin badly, and CRI is where that shows up. Below 90, be careful with faces.
Cyc: A curved, seamless wall-to-floor surface with no visible corner, so the background reads as infinite space. What people mean when they ask for a “white background” with no horizon line.

Grip and rigging

None of this appears in the image. All of it decides whether the image is possible, and whether anybody gets hurt.

C-stand: The century stand: three legs of different heights that nest against a wall, and an arm that holds a flag or a small lamp. Rule that survives everyone: the tallest leg goes under the arm, and the sandbag goes on that leg.
Apple box: A wooden box in four standard sizes, used to raise an actor, level a table leg, seat a crew member or take a camera's weight. Named by size — full, half, quarter, pancake — and used constantly.
Sandbag: The reason the stand does not fall over. On any stand carrying weight, in any room, indoors included: one cable caught by one foot is all it takes.
Matte box: Fitted to the front of the lens: holds filters and shades the glass from stray light. Its hood does more for contrast than most filters inside it.
Dolly: A wheeled platform for the camera, on track or on the floor. It moves the audience's point of view through space, which is not the same thing as a zoom: a dolly changes perspective, a zoom only magnifies.
Gimbal: A motorised stabiliser that keeps the camera level while the operator walks. It removes shake, and it also removes the weight and hesitation that make a handheld shot feel human. Both are decisions.
Grid: The framework of pipes above a studio floor from which lamps and backdrops are hung. Grid height, more than floor area, is what decides which lighting is possible in a room.

Camera and image

The settings that change what the picture is, rather than how bright it happens to be.

Stop: One stop is a doubling or halving of light. It is the unit everything in exposure is counted in: aperture, shutter, ISO and filters all trade against each other in stops. Learn to think in stops and exposure stops being arithmetic.
T-stop vs f-stop: An f-stop is geometry — focal length divided by pupil diameter. A T-stop is measured transmission, accounting for the light the glass absorbs. Cinema lenses are marked in T-stops so that two different lenses at T2.8 actually expose the same. Stills lenses are marked in f-stops and do not.
Shutter angle: How much of each frame's duration the sensor is exposed for, expressed as an angle from the days of rotating disc shutters. 180° means half the frame, which is the motion blur a century of cinema has trained everyone to read as film. Work out the shutter speed for any angle and frame rate.
ND filter: Neutral density: takes light out in front of the lens without changing color. It is how you keep a 180° shutter and a wide aperture in daylight, which is the only way to get cinematic depth of field outdoors. Which ND you need.
Log: A recording curve that stores highlights and shadows across a much wider range than the screen can show, at the cost of looking flat and gray out of camera. It is not a look, it is a container — log footage is meant to be graded, and judging it ungraded tells you nothing.
LUT: A look-up table that maps one set of colors onto another. A technical LUT converts log to a normal viewing curve; a creative LUT applies a look. Used on set for monitoring so nobody has to imagine what the flat image will become.
Raw: Sensor data recorded before demosaicing and before white balance is baked in. It keeps decisions open until post, and costs a great deal of card space and processing. Raw is not sharper — it is more negotiable.
Codec: The scheme that compresses the picture into a file. It decides file size, how heavy editing will be, and how much the image survives being graded. What bitrate each platform actually asks for.
Bitrate: Data per second, in megabits. More bits means fewer compression artefacts and bigger files. Beyond a certain point the extra bits buy nothing visible, and where that point falls depends on the codec, not on the number.
Dynamic range: The span between the darkest and the brightest detail a sensor can hold in one exposure, counted in stops. It is what decides whether a window in shot goes white or keeps its view.
False color: A monitoring mode that paints the image in flat colors by exposure level, so you can see at a glance what is clipping and where skin sits. Far more reliable than judging brightness on a monitor in daylight.
Rolling shutter: Most sensors read out line by line rather than all at once, so fast pans and quick movement lean or wobble. It is a property of the sensor: you work around it with slower moves, not with a setting.
Anamorphic: Lenses that squeeze a wide image onto the sensor, unsqueezed in post. They give the oval highlights, horizontal flares and particular focus fall-off associated with widescreen cinema — and they cost light and focus speed.
Crop factor: How much smaller a sensor is than 35 mm full frame, and therefore how much narrower a given lens looks on it. It changes the field of view, never the focal length. What a lens actually covers on your sensor.
Circle of confusion: The largest blur spot still read as a point, and the number every depth of field calculation quietly depends on. It is a convention, not a physical constant — which is why two calculators can disagree and both be right. Depth of field, with the value it uses shown.
Aspect ratio: The shape of the frame, width over height. Changing it after the shoot does not resize the picture, it throws part of it away. How much a reframe costs.

Sound

An audience forgives a soft picture far longer than it forgives bad sound. This is the department that gets cut first and should not be.

Lav: Lavalier: the small microphone clipped to clothing or hidden under it. Consistent level, close sound, and a permanent fight with fabric rustle and jewellery.
Room tone: Thirty seconds to a minute of the empty room, recorded on every location before you leave. The editor uses it to fill gaps and smooth joins. It costs a minute on the day and cannot be recreated afterwards.
Wild track: Sound recorded without picture — a line re-read, an atmosphere, a specific noise. Not synced to anything, and often what saves a scene in the edit.
Timecode and jam sync: A clock shared by every device so that sound and picture line up without hunting for claps. Jamming is the act of copying that clock from a master to each recorder. Read and convert timecode, drop frame included.
Phantom power: 48 volts sent down the cable to power a condenser microphone. Switched on at the recorder, and the reason a good microphone stays silent when someone forgets.
MOS: A shot filmed without sound. The initials have a dozen disputed origin stories and none of them matter — on a slate it simply means nobody should go looking for the audio.
LUFS: The unit of perceived loudness used by every streaming platform to play everything at a comparable volume. It, not peak level, decides how loud your mix reaches the listener. Measure it, and see the penalty each platform applies.

Shooting and production

The vocabulary of the day itself: what gets planned, what gets called, and what gets promised.

Call sheet: The document sent the night before: who arrives when and where, what is being shot, sunrise, sunset, weather, hospital address. If it is wrong, the day is wrong before it starts.
Coverage: The set of angles that let a scene be cut together: wide, mediums, close-ups, reverses. Insufficient coverage is only discovered in the edit, when it is expensive.
B-roll: Everything that is not the main action: cutaways, details, atmosphere. It carries the edit, hides cuts, and is the first thing dropped when a day runs late — then missed for the rest of the project.
Plate: A clean shot, without the subject, kept for effects work: an empty background, a passing street, a sky. Costs thirty seconds on the day and saves days in post.
Packshot: The shot of the product itself, lit and framed for its own sake. Usually the reason the commercial exists, and usually scheduled last, when the light and the crew are tired. Schedule it first.
Pickup: A shot filmed after the main shoot to fill a gap found in the edit. Cheaper than a reshoot, harder to match than anyone expects.
Slate: The clapperboard: identifies the shot and gives sound and picture a common point to sync on. Even with timecode it is still filmed, because it is the one label a human can read on the footage itself.
Martini shot: The last shot of the day. Announcing it is a promise, and the crew will remember whether it was kept.
Golden hour: The hour after sunrise and before sunset: low, warm, soft light. It is not an hour — it is closer to thirty minutes, and it does not wait. Everything else is prepared before it starts.
Day for night: Shooting in daylight and grading to look like night. It fails when the sky is in shot and when shadows are wrong, and it works when the frame is tight and the exposure is planned for it from the start.
Continuity: Making sure that what was true in one shot is still true in the next: the level in the glass, the cigarette length, which hand the keys are in. The audience never notices it done well and always notices it done badly.

Frequently asked

What is the difference between a gaffer and a key grip?

The gaffer is responsible for everything that emits light — lamps, power, dimming. The key grip is responsible for everything that shapes or supports it: flags, diffusion frames, stands, dollies, rigging. One adds light, the other takes it away and holds it in place. On a large set they lead two separate departments and work as a pair.

What does 180 degree shutter mean?

That the sensor is exposed for half of each frame's duration. The name comes from the rotating disc shutters of film cameras, where a half-open disc gave exactly that. At 24 fps it is 1/48 s. It is the amount of motion blur audiences have learned to read as cinema — narrower looks stroboscopic, wider smears.

What is the difference between an f-stop and a T-stop?

An f-stop is calculated from geometry: focal length divided by the diameter of the entrance pupil. A T-stop is measured, and accounts for the light the glass absorbs. Two lenses set to f/2.8 can expose differently; two lenses set to T2.8 will not. That is why cinema lenses are marked in T-stops and stills lenses are not.

Why do people record room tone?

Because silence is never silent, and every room's silence is different. The editor needs that specific background to fill pauses, bridge cuts and cover replaced dialogue. It takes a minute on location and is impossible to fabricate afterwards — which is why leaving without it is one of the classic regrets of post-production.

What does MOS mean on a slate?

That the shot was filmed without sound. The origin of the initials is disputed — the popular story about a German-speaking director is almost certainly apocryphal — and it does not matter: in practice it tells the assistant editor not to go looking for audio that was never recorded.

Is negative fill really useful?

It is often the only thing that works. Outdoors on a bright overcast day, or in a small white room, light is arriving from every direction at once and the face has no modeling at all. You cannot fix that by adding a lamp — you fix it by putting something black on one side. It is the cheapest piece of equipment on the truck and the one that changes the most.

The film set glossary