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How To Use A Footcandle Meter For Photography

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The output of LED lights is rated in lux, lumens or footcandles. What the heck exercise all of these terms mean when all nosotros want to know is how bright the light is in f/end, shutter speed and ISO? Here are the simple answers …

Footcandles are for movies, video and television receiver

I call up working in a TV news studio before in my career, we had to become at to the lowest degree 300 fc — the abbreviation for footcandles — on the set for it to annals on the cameras. I walked around the gear up with an incident low-cal meter looking for hot spots and nighttime ones. Hot spots were reduced past changing to a lower wattage bulb or calculation some fireproof diffusion to the light. College watt bulbs boosted the dark spots to proper levels.

Footcandles are existence replaced past 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 past dividing by 10. Converting footcandles to lux is done past multiplying past 10 for a ballpark respond. 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 desire to know is the aperture and shutter speed to use at a given ISO for a light. The cardinal to this is simple: 1000 lux or 100 fc give an exposure of f/ii.viii at 1/50 for ISO 100. That info is all that yous need to figure out any exposure for any amount of lux or fc.

Exposure is doubling or cutting light in half

Hither are some basics in example you don’t take 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 e’er double the corporeality of lite or half the amount depending on whether yous open up up or terminate downwardly.

  • Open up i f/stop doubles the corporeality of light
    • Motion from f/two.8 to f/2.0 doubles the amount of calorie-free
    • Move from 1/50s to 1/25s doubles the amount of lite
    • Move from ISO 100 to ISO 200 doubles the amount of light
  • Close downward ane f/stop cuts the amount of light in half
    • Motion from f/2.8 to f/4.0 halves the amount of calorie-free
    • Movement from i/50s to 1/100s halves the corporeality of light
    • Move from ISO 100 to ISO 5o halves the amount of low-cal

Let’s utilize the 1000 lux/100 fc exposure of f/2.eight at 1/l at ISO 100 to meet how this works.

  • When 1 command (aperture, shutter speed or ISO) is opened upward one f/stop, 1 of the other controls must be closed downwardly one f/end.
    • Open up the aperture ane f/stop: f/2.8 moves more open to f/two.o (doubles the corporeality of lite) at ISO 100
    • Close the shutter speed i f/cease: 1/50s moves faster to 1/100s (halves the corporeality of light) at ISO 100
      • f/2.0 at 1/100s and ISO 100 is the same exposure as f/two.8 at 1/50s and ISO 100

How bright is that light? Your guide to lux, footcandles and exposureExposure according to lux or fc

The example above is for one thousand lux (100 fc). Increase the amount of light to 4000 lux (400 fc) and the difference is 2 f/stops (or just stops) more light.

Information technology works this manner: 1000 lux (100fc) doubles to 2000 lux (200fc). That is 1 stop brighter. Double 2000 lux (200fc) to 4000 lux (400fc) and the brightness increases by another stop for a total of 2 stops more calorie-free. So …

At 4000 lux (400fc) the exposure is ii stops more light and is controlled past stopping down by 2 f/stops.

  • The original exposure for chiliad lux (100 fc) is however f/2.viii at 1/50 ISO 100
  • At 2000 lux (200 fc) there is twice as much low-cal. The exposure is now
    • f/4.o at ane/50s and ISO 100 or
    • f/2.eight at 1/100s and ISO 100 or
    • f/2.8 at i/50s and ISO 5o
      • Each of the higher up is the correct exposure for 2000 lux (200 fc).
  • At 4000 lux (400 fc) there is double the corporeality of light from 2000 lux (200 fc)
    • f/5.6 at 1/50s and ISO 100 or
    • f/2.eight at 1/200s and ISO 100 or
    • f/two.8 at 1/50s and ISO 25 or
    • f/4.0 at ane/100s and ISO 100 (both the aperture and shutter accept closed down one stop).

Reading a light’s output

I recently reviewed a new spot/overflowing bicolor LED light from Genaray. The BLT-60B‘s specs listing the following super hard to read nautical chart but now you accept the hush-hush decoder band!

15° spot:
9050 lux/840.seven fc at ii.99′ / 0.91 m
75° flood:
3270 lux/303.viii fc at 2.99′ / 0.91 m

Lite is measured in lux and fc at a distance

In the specs above the lux and fc are measured at a super shut to three feet. Permit’s go over the first line to come across how this conversion works.

I desire to know how many stops brighter 9050 lux is than grand lux. (To continue 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 (one-half the lite or one stop) = 4525 lux
  • 4525 lux divided by 2 (half the low-cal over again is one more finish) = 2250 lux and yes I am rounding for simplicity’southward sake.
  • 2250 lux divided by 2 (one-half over again is yet another finish) = 1125 lux and then

The exposure for 9050 lux at three feet is f/two.8 at one/400 ISO 100.

You lot can work out the 75º flood line because now y’all know how.

All y’all have to remember is

m lux (100 fc) equals an exposure of f/two.viii at 1/50 at ISO 100. And that lux or footcandles are ever measured at a stated altitude from the light.

Blow out the candle. We’re done here.

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Jeff Stephens

I worked in the lighting industry more xxx years, I never used Pes candles ever, that is such an outdated measurement. It is an imperial measurement, nothing to practice with the film industry. With reference to a low-cal source’s output, a light sources output is measured in lumens. The light it gives at a certain altitude is measured in lux. So taking your example adding, the figures you quote only stand up if you are 0.91m from the source. The lux value diminishes by changed square low for every extra 0.91m you lot are away from the source. Again this is

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Emmanuel Ramses

This was exactly my question when reading: how does the inverse square constabulary apply? Specifically, whether we stick to meters when figuring output.

Kevin Ames

The changed square constabulary is more math than I want to practise. Basically, the farther a subject field gets from a light source the less low-cal 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 lite has to cover the dimmer it volition be.
I
e’er
utilise my meter to determine the subject area’s exposure and the reflectance of the background.

Greg

A very helpful article, thank you.

Neil

All I demand to know is the watt seconds! I instantly know what I need to practise with no bloody maths!!!!
I’m as thick equally, and hate all this lux blah blah…. 🙂

Kevin Ames

Hi Neil,
I hear ya. Affair is, that continuous lights like LEDs aren’t rated in watt-seconds. So a bones understanding of footcandles, lux and lumens are needed to make an informed decision when choosing one of these lights.

Tim Walker

Adept simple explanation that makes simple comparisons possible. Many thanks.

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