In short: Light from a point source falls off with the square of the distance, so doubling the distance cuts the light to a quarter. Lux measures the light landing on a surface. T-stop measures the light a lens passes. Measure the lux at the subject, then choose the distance and the T-stop that give the exposure you want.
What the calculator does
The calculator has two parts. Part 1 takes the lux a fixture gives at a reference distance, which comes from its photometric data sheet, and works out the lux at the distance you choose. Give it a lux you want and it works out the distance that gives it. Part 2 works out the T-stop for a given amount of light, or the light needed for a given T-stop, from the ISO, the shutter speed and the meter calibration constant (250 by default, which you can change to match your meter). It shows foot-candles too. [[asset]]
The inverse square rule
The intensity of light from a point source is inversely proportional to the square of the distance from the source. Doubling the distance cuts the light on the subject to a quarter, and halving the distance makes it four times brighter.
The rule assumes a point source. A large softbox or a wide fresnel is not a point at close range, so the sum is a starting point. Take a reading at the subject before you lock the position.
Lux and what it measures
Lux is the SI unit of illuminance. One lux is one lumen falling on one square metre. The older foot-candle is about 10.76 lux. A typical TV studio is about 1,000 lux, similar to an overcast day, and an office is about 320 to 500 lux.
For a shoot, lux describes the light on the subject, not the light coming out of the fixture. Take the reading at the height and angle of the face or the product, with the lights you plan to use switched on.
T-stop and f-number
The f-number is the focal length divided by the diameter of the lens opening. It says nothing about the light the glass passes. The T-stop starts from the f-number and corrects for transmission: divide the f-number by the square root of the transmittance. A lens at f/2.0 that passes 75 per cent of its light has a T-stop of about 2.3.
Cinema lenses are marked in T-stops because a cut between two lenses has to match. Still photographers mostly work with f-numbers.
A worked example
Say a fixture gives 800 lux at one metre from the subject. Using the inverse square rule:
- At 2 metres the light is a quarter of 800, so about 200 lux.
- At 4 metres it is a sixteenth of 800, so about 50 lux.
- To get 400 lux, the fixture needs to be about 1.4 metres away, because 800 divided by the square of 1.4 is roughly 400.
This works when the fixture is the main source. Light from other fixtures, or bounce from the walls, adds to the reading, so take it with the whole set lit as it will be shot.
How to use it on a shoot
- Decide the look and the lux you want on the subject.
- Choose the lens and the T-stop for the shot. Check the frame with the field of view calculator.
- Work out the distance for the light with part 1 and mark it on the floor, then check the T-stop with part 2 at your ISO and shutter speed.
- Take a reading at the subject with the lights you will use, then adjust the distance or the output.
- Check the picture on the monitor with the actor in place, not on a chart.
Common mistakes
- Using a lux figure from one fixture at one distance as if it holds at every distance.
- Mixing up f-number and T-stop when matching two lenses for a cut.
- Taking the reading with the room lights off and missing the practical lamps in shot.
- Treating the inverse square sum as exact for a large softbox at close range.
Where this fits
The lighting plot planner puts each fixture on the grid, and the colour temperature and gel cheat sheet covers the colour side of the same decision. The White Space has an 8-bar grid 4.5 metres above the floor. If you would rather hire the room and a gaffer than work the sums yourself, see gaffers and lighting crew.
Sources
- Wikipedia, Inverse-square law, read 5 November 2026
- Wikipedia, Lux, read 5 November 2026
- Wikipedia, T-stop, read 5 November 2026
Related guides
Questions we get asked
How much light do you lose when you double the distance?
Light from a point source falls to a quarter when the distance doubles, because the same light spreads over four times the area. A fixture at two metres gives a quarter of the light it gives at one metre.
What is the difference between an f-number and a T-stop?
The f-number compares the lens opening with its focal length and ignores the light lost in the glass. The T-stop corrects for that loss, so it is the better guide for matching exposure between lenses in a sequence.
How many lux do you need for a studio shot?
There is no single figure. A typical TV studio is about 1,000 lux, but the right level depends on the camera, the lens and the look you want. Measure the subject with the lights you will use.
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