and then there was the footcandle meter.

and so the footcandle meter and it's mysterious footcandles...

Yeah... I dunno.

This very cool old foot-candle meter was in Andy's stuff. He got it in a trade or something... maybe a pile of stuff from a relative. It has two ranges, actuated with a side mounted switch, and runs on solar power, like it should. No need for batteries.. ever.

Anyway, it works, and works well! There's something satisfying about an old needle meter that is accurate, especially if it is encased in stale gray bakelite, or brittle plastics of the past.

Oh yeah, about these mysterious foot-candles:

First off, to make sure we are all totally confused here: 1 footcandle = 10.76391 lux, and none of this has much to do with the brightness of a lightbulb.

Allow me to explain.

A foot candle is a measure of "illuminance." Notice I didn't say "Luminance." It's a measure of the light that is hitting something. Namely, your subject. It doesn't measure how powerful a lightbulb is, because as you get the subjet farther away from the light source, less light is hitting the subject. the "illuminance" decreases. The subject is less "illuminated." Not that they are getting any dumber or anything. It's all about the destination, and not anything about whatever source of light. Get it? You can't possibly say that this new flashlight puts out ten foot candles right?

That makes this meter useful for photography, lighting design... all sorts of things becasue we are all taking pictures of stuff, not the lights that light it.

So, how much light is a "foot candle" and how can that be converted to something useful that has nothing to do with people's feet?

The original idea was that one foot-candle was the amount of light that would hit the inside of a one foot radius sphere if, yes, there was a one candela source of light (basically a candle,) in the middle. No, really. a two foot diameter beach ball with a candle in the middle? Anywhere on the inside of the beach ball would be one foot-candle of light. Pretty simple.

Whatever.

So it's the amount of light that hits a surface. So you could get one "foot candle" of light with one candle a foot away from you, or, say four candles two feet away. or sixteen candles four feet away. Sixteen candles. heh. Remember two things:

ONE, we are measuring the light hitting the subject, not the amount that a candle can make in total or anything weird like that.

TWO, as with anything, inverse square law applies. Maybe I'll further digress with a quick diagram.

See? It's all about the "density" of the light.
Twice as far is the same light covering twice the height and twice the width, so you get a quarter candle per foot at twice the distance. Inverse square law!

Sorry about being redundant, off the subject and and referring to ridiculous movies, but this is MY blog so I can cry if I want to.

There's a metric equivalent to all this that's in meters and all that. It's called LUX. Much sexier name, and being metric, pretty much obsoletes foot-candles, although people still use foot-candles on things here in the US.

They are measuring the same thing, so one footcandle is simply 10.764 lux. When I worked in lighting, we just multiplied by ten and that was close enough.

Oh, and blasting sunshine is about 10,000 foot-candles anywhere here on earth.

What about camera settings?

Assuming 100 speed film or setting on your digicam nonsense:

10,000 FC(foot-candles) should be almost f/16 at a 250th of a second.

100 FC would be what? 250th at f/1.4 or so.

1 foot candle is about f/2.0 at a one second exposure.

maybe a chart later.

You can figure it out. Either that or get a hold of me and I'll scorn you for not having it figured out.

This is nuts. I'm going to bed.