Inverse Square Law Light Falloff Calculator for Film Sets

The inverse square law describes how the intensity of light from a point source decreases as distance from that source increases. The mathematical relationship is that intensity is inversely proportional to the square of the distance. If you double the distance, you get one quarter of the light. Triple the distance and you get one ninth. In film and television production, this law governs every lighting decision you make. It explains why the background goes dark behind a subject lit with a nearby flash, why you lose exposure when you push a key light back to clear a camera move, and why a small change in actor position near a close key light creates a significant exposure shift that requires a stop adjustment from your AC.

Not precisely, at least not at close distances. The strict ISL formula applies to true point sources, which are light sources that are physically small relative to the distance at which you are measuring. Large modifiers such as softboxes, octaboxes, 4×4 silks, and large LED panels do not behave as point sources at close distances. At distances less than five times the modifier’s largest dimension, the falloff is shallower than the ISL predicts, approaching linear falloff (intensity dropping proportionally to 1/d rather than 1/d squared) very close to the source. Beyond the five-times-diameter threshold, the source effectively becomes a point source and the ISL applies normally. This calculator’s soft source mode accounts for this using an empirical correction factor based on the ratio of shooting distance to modifier size.

Footcandles and lux are both units of illuminance, which measures how much light falls on a surface. The difference is the unit area: one footcandle is one lumen per square foot, while one lux is one lumen per square meter. The conversion is exactly 1 fc = 10.7639 lux, per the NIST standard. In the United States, professional film and television production traditionally uses footcandles because the light meters most commonly found on set (particularly the Sekonic L-758 and L-858 series) display footcandles by default. Many European and international productions use lux. This calculator accepts both and converts automatically, so use whichever unit your light meter displays. The physics is identical regardless of the unit.

A T-stop (transmission stop) is a calibrated measure of actual light transmission through a specific lens at a specific aperture setting. Unlike an f-stop, which is a purely geometric ratio, a T-stop accounts for light lost to refraction, reflection, and absorption within the lens elements. On a cinema set, cameras are set to T-stops because they give a more accurate exposure prediction than f-stops. This calculator derives the T-stop from the footcandle reading at your target distance using the standard incident metering formula: N squared equals fc times shutter time times ISO divided by 322, where 322 is the calibration constant used by Sekonic for incident footcandle metering. The result is the aperture value N, reported as a T-stop. This is an approximation because the formula does not account for individual lens transmission curves, but it is accurate enough for planning purposes to within plus or minus about a third of a stop.

Under OSHA 29 CFR 1926.56, Table D-3, the minimum illumination for general construction areas, which includes film and television location sets, is 5 footcandles. The same standard requires 10 footcandles for shops and active work areas, and 30 footcandles for first aid stations and areas with detailed visual tasks. These are minimum safety requirements, not lighting design targets. As of 2025, OSHA can issue fines of up to $16,550 per serious violation. On a large production with many crew members working in underlit areas, multiple citations can accumulate quickly. The OSHA compliance indicator in this calculator checks whether your calculated intensity at the target distance meets the 5 fc general area minimum as a quick reference during pre-production planning.

An exposure value (EV) is a measure of photographic exposure that combines aperture and shutter speed into a single number. A change of 1 EV, or 1 stop, represents a doubling or halving of the amount of light reaching the sensor. In the context of the inverse square law, an EV change tells you how many stops the exposure changes when your subject moves from the reference distance to the target distance. A negative EV change means the target is darker than the reference, a positive EV change means it is brighter. The formula for EV change due to distance alone is 2 times the log base 2 of d2 divided by d1. Every time the distance doubles, the EV changes by exactly 2 stops. Cinematographers use this when planning actor blocking so they know in advance how much the exposure will shift as the actor moves through the space.

Background separation is the difference in exposure, measured in stops, between the subject and the background. In this calculator, the background is assumed to be at twice the target distance from the light source, which is a common approximation for a standard single-light setup. For example, if your subject is 8 feet from the key and the background is 16 feet from the key, the background receives exactly 2 stops less light than the subject. Background separation matters because it is the primary tool cinematographers use to create visual depth and avoid the flat, even look of a badly lit scene. A 2-stop separation is considered a natural, motivated look. More than 3 stops creates dramatic separation. Zero stops of separation looks like a surveillance camera. The exact number depends on the scene, the DP’s style, and the director’s visual intent.

Yes. The inverse square law applies equally to continuous light sources (like HMIs, tungsten, LEDs) and strobe or flash sources. The physics is the same: intensity falls off with the square of the distance regardless of whether the source is pulsed or continuous. The only difference is how you measure and express the initial intensity. For continuous lights, you use a footcandle or lux reading from an incident meter. For strobe, you would typically express the initial intensity at a reference distance from your flash guide number or from a meter reading at a known flash power setting. Enter the reading at your reference distance and the calculator handles the rest. The T-stop output is only relevant for shutter-speed-dependent exposures, so for strobe work it is less useful, but the fc, lux, and EV outputs are fully applicable.

This is a question that cinematographers get wrong all the time because they conflate quality with intensity. A softbox and a fresnel can output the same footcandle reading at the subject, but the softbox creates softer shadows and transitions because it is a large source relative to the subject. The softness is a product of source size relative to subject size, not intensity. The inverse square law is still at work in both cases, but the larger source creates a more gradual shadow edge because light arrives at the subject from a wider range of angles. The soft source falloff correction in this calculator matters when you are trying to predict intensity at multiple distances, not when you are trying to evaluate whether soft or hard light is more appropriate for a specific shot. Those are separate creative and technical decisions.

The fixture presets use approximate photometric values derived from manufacturer specification sheets and published performance data. These values represent typical output in peak spot or flood position and should be treated as starting points for your calculation, not as precise measurements. Actual output will vary based on the specific unit, its age, the fresnel lens position, any diffusion material, and the ambient temperature of the operating environment. Older tungsten units lose efficiency as the lamp approaches the end of its life. HMI output can vary by 20 to 30 percent across the lamp’s life cycle. The correct approach is always to measure the actual output with a calibrated incident light meter at your specific reference distance on set, then use that reading in the calculator. The presets are useful for pre-production planning when you do not yet have the fixture in hand.

For standard US narrative film and television production at 24 frames per second, the most common settings are ISO 800 with a 180-degree shutter angle (giving an exposure time of 1/48 second). This is the starting baseline for most ARRI Alexa and RED camera systems on American productions. High-end narrative features often shoot at ISO 400 for a cleaner image at the cost of needing more light. Television episodic productions frequently push to ISO 1600 or 3200 on location to reduce the lighting package. Commercials and music videos sometimes use ISO 200 for maximum image quality when controlling the light is easier. The 172.8-degree shutter is preferred on some productions to avoid any potential flicker with 60Hz US power at 24fps, though modern LED fixtures with flicker-free drivers eliminate this concern at 24fps.

No. The inverse square law describes only the direct light from the source to the subject, not the bounced or reflected light that fills in shadows and reduces contrast in a real room. In a small, white-walled room, reflections can add a significant amount of fill that is not predicted by the ISL. On a film set, where walls are often covered with black duvetyne or flags are used to control reflections, the ISL is a more accurate predictor of actual intensity because the bounce contribution is actively controlled. The formula published by researcher and photographer Gary Friedman in a 2025 blog post confirms that the ISL does not apply to the light bounced from the subject back to the camera. A dark backdrop behind the subject at 16 feet will read significantly darker than the ISL predicts for a bare room, because there is no bounce contribution. In outdoor settings with no surrounding surfaces, the ISL is essentially exact for point sources.

Geometric dilution is the physics explanation for why the inverse square law exists. Imagine a light source radiating energy equally in all directions. At any given distance from the source, that energy is spread over the surface of an imaginary sphere. The surface area of a sphere grows with the square of its radius. When you double the radius, the surface area quadruples. Because the total energy from the source is constant, and that energy is being spread over four times the area, the intensity per unit area drops to one quarter. That is exactly what the ISL describes mathematically. The source is not losing energy. The energy is being diluted across a larger and larger sphere as you move away from it. This is also why the ISL applies to gravity, radiation, sound, and any other energy that radiates from a point in three-dimensional space.

Yes, this is one of the most practical applications of the ISL calculator on a US television or commercial set. When two cameras are at different distances from the key light, or when the same scene is lit from different angles for A and B camera coverage, the inverse square law tells you exactly how much the exposure will differ between setups. Run the calculation twice: once with the A camera subject distance and once with the B camera subject distance. The difference in footcandles tells you whether you need to adjust ISO, add a net on the key, move the light, or bounce additional fill to match the two exposures before you commit to the setup. Doing this in pre-production using this tool is faster and more reliable than discovering the mismatch during the grade when the production has already moved on.

No. The inverse square law is a consequence of geometry, not of the light source technology. It applies equally to LED, tungsten, HMI, fluorescent, and any other light source, provided that source approximates a point source at the relevant distances. Where LEDs differ from tungsten and HMI is in their photometric behavior at extreme dimmer levels, their color rendering across the dimming range, and their flicker behavior at different power levels. The ISL itself does not change. However, many LED panels are large array sources rather than point sources, which means the soft source correction in this calculator is relevant for LED panels at close distances. A compact LED puck or a single LED module in a fresnel housing behaves as a point source. A 12-inch by 12-inch LED panel does not, at least not at distances closer than about 5 feet.

The background separation output in this calculator gives you the stops difference between the subject and the background assuming a single source setup where the background is at twice the subject distance from the key. For a full three-point setup with a separate background light, you would run the calculation separately for the background fixture. Enter the background light’s intensity at its reference distance, then set the target distance to the background surface. The resulting footcandle reading tells you the background exposure. Compare that to your key light reading at the subject to determine the ratio. Most US productions target a 1 to 2 stop difference between subject and background for a natural look, or 2 to 4 stops for a dramatic, high-contrast style. The PDF report from this calculator can document both calculations side by side for your pre-production planning file.