The output of LED lights is rated in lux, lumens or footcandles. What the heck do all of these terms mean when all we want to know is how bright the light is in f/stop, shutter speed and ISO? Here are the simple answers …
Footcandles are for movies, video and television
I remember working in a TV news studio earlier in my career, we had to get at least 300 fc — the abbreviation for footcandles — on the set for it to register on the cameras. I walked around the set with an incident light meter looking for hot spots and dark ones. Hot spots were reduced by changing to a lower wattage bulb or adding some fireproof diffusion to the light. Higher watt bulbs boosted the dark spots to proper levels.
Footcandles are being replaced by lux
Lux is the European equivalent of footcandle and is slowly replacing footcandles. Unfortunately, they aren’t equal. There are 10.74 lux to one footcandle. Roughly, people convert lux to footcandles by dividing by 10. Converting footcandles to lux is done by multiplying by 10 for a ballpark answer. This is close enough for quick exposure calculations.
Lux, footcandles and the exposure triangle
Lux and footcandles are so not useful to photographers. What photographers want to know is the aperture and shutter speed to use at a given ISO for a light. The key to this is simple: 1000 lux or 100 fc give an exposure of f/2.8 at 1/50 for ISO 100. That info is all that you need to figure out any exposure for any amount of lux or fc.
Exposure is doubling or cutting light in half
Here are some basics in case you don’t have a total handle on how exposure works. First, remember that an f/stop — whether referring to an aperture value, shutter speed or an ISO — is always double the amount of light or half the amount depending on whether you open up or stop down.
- Open up one f/stop doubles the amount of light
- Move from f/2.8 to f/2.0 doubles the amount of light
- Move from 1/50s to 1/25s doubles the amount of light
- Move from ISO 100 to ISO 200 doubles the amount of light
- Close down one f/stop cuts the amount of light in half
- Move from f/2.8 to f/4.0 halves the amount of light
- Move from 1/50s to 1/100s halves the amount of light
- Move from ISO 100 to ISO 5o halves the amount of light
Let’s use the 1000 lux/100 fc exposure of f/2.8 at 1/50 at ISO 100 to see how this works.
- When one control (aperture, shutter speed or ISO) is opened up one f/stop, one of the other controls must be closed down one f/stop.
- Open the aperture one f/stop: f/2.8 moves more open to f/2.o (doubles the amount of light) at ISO 100
- Close the shutter speed one f/stop: 1/50s moves faster to 1/100s (halves the amount of light) at ISO 100
- f/2.0 at 1/100s and ISO 100 is the same exposure as f/2.8 at 1/50s and ISO 100
Exposure according to lux or fc
The example above is for 1000 lux (100 fc). Increase the amount of light to 4000 lux (400 fc) and the difference is 2 f/stops (or simply stops) more light.
It works this way: 1000 lux (100fc) doubles to 2000 lux (200fc). That is one stop brighter. Double 2000 lux (200fc) to 4000 lux (400fc) and the brightness increases by another stop for a total of 2 stops more light. So …
At 4000 lux (400fc) the exposure is two stops more light and is controlled by stopping down by 2 f/stops.
- The original exposure for 1000 lux (100 fc) is still f/2.8 at 1/50 ISO 100
- At 2000 lux (200 fc) there is twice as much light. The exposure is now
- f/4.o at 1/50s and ISO 100 or
- f/2.8 at 1/100s and ISO 100 or
- f/2.8 at 1/50s and ISO 5o
- Each of the above is the correct exposure for 2000 lux (200 fc).
- At 4000 lux (400 fc) there is double the amount of light from 2000 lux (200 fc)
- f/5.6 at 1/50s and ISO 100 or
- f/2.8 at 1/200s and ISO 100 or
- f/2.8 at 1/50s and ISO 25 or
- f/4.0 at 1/100s and ISO 100 (both the aperture and shutter have closed down one stop).
Reading a light’s output
I recently reviewed a new spot/flood bicolor LED light from Genaray. The BLT-60B‘s specs list the following super hard to read chart but now you have the secret decoder ring!
15° spot: 9050 lux/840.7 fc at 2.99′ / 0.91 m
75° flood: 3270 lux/303.8 fc at 2.99′ / 0.91 m
Light is measured in lux and fc at a distance
In the specs above the lux and fc are measured at a super close to three feet. Let’s go over the first line to see how this conversion works.
I want to know how many stops brighter 9050 lux is than 1000 lux. (To keep it simple, I am dropping the footcandles for now. If you want to use them, the question is how many stops brighter are 840 fc than 100 fc?)
- 9050 lux (840 fc) divided by 2 (half the light or one stop) = 4525 lux
- 4525 lux divided by 2 (half the light again is one more stop) = 2250 lux and yes I am rounding for simplicity’s sake.
- 2250 lux divided by 2 (half once more is yet another stop) = 1125 lux so
The exposure for 9050 lux at 3 feet is f/2.8 at 1/400 ISO 100.
You can work out the 75º flood line because now you know how.
All you have to remember is
1000 lux (100 fc) equals an exposure of f/2.8 at 1/50 at ISO 100. And that lux or footcandles are always measured at a stated distance from the light.
I worked in the lighting industry more than 30 years, I never used Foot candles ever, that is such an outdated measurement. It is an imperial measurement, nothing to do with the film industry. With reference to a light source’s output, a light sources output is measured in lumens. The light it gives at a certain distance is measured in lux. So taking your example calculation, the figures you quote only stand if you are 0.91m from the source. The lux value diminishes by inverse square low for every extra 0.91m you are away from the source. Again this is… Read more »
This was exactly my question when reading: how does the inverse square law apply? Specifically, whether we stick to meters when figuring output.
The inverse square law is more math than I want to do. Basically, the farther a subject gets from a light source the less light is falling on it. That’s because the greater the distance of the subject to the light the more the light spreads out. The wider area the light has to cover the dimmer it will be.
I always use my meter to determine the subject’s exposure and the reflectance of the background.
A very helpful article, thank you.
All I need to know is the watt seconds! I instantly know what I need to do with no bloody maths!!!!
I’m as thick as, and hate all this lux blah blah…. :)
Hi Neil,
I hear ya. Thing is, that continuous lights like LEDs aren’t rated in watt-seconds. So a basic understanding of footcandles, lux and lumens are needed to make an informed decision when choosing one of these lights.
Good simple explanation that makes simple comparisons possible. Many thanks.