How do I interpret flashlight specs for light painting at a distance?

Context

So I've seen light painting shots and I'd like to try it out sometime. I'm more interested in painting say parts of the landscape so they stand out during a long exposure.

Question

Is there any candela equation for finding out given the amount of candela how much reach it will have? If no, are there recommendations or related for that? Do manufacturers actually post valid results or are they also fudged? Are there specific features that should be considered when shopping?

I assume for features I'd need to look at battery life and what throws it has but doesn't tell me how far the light beam can necessarily travel. Of course this changes in fog or other environments but in my case I'm aiming for a clear, or clear enough night.

Related

How do I get started with 'painting with light' photography?

• Not specific to photo purposes, but this site has a lot of information about the variations in how light gets thrown, how meaningful the numbers really are, etc... Dec 7, 2015 at 22:11
• I don't know an answer about equations, but strongly suspect any numbers will just be a confusion. I think you're trying to overthink it. Because in any case, you'll still have to experiment a bit to find a useful exposure. More exposure could result from a larger light, or a closer light, or from a wider aperture, or a higher ISO, or a longer exposure time of the light, or some of all of them. So just get a decent light, and go out and wade in. Try some things. You'll learn much more in ten minutes there than in days here. Dec 7, 2015 at 22:45
• As an extreme endpoint for calculation, here's a 90,000 lumen flashlight. Probably a bit overkill.
– user13451
Dec 7, 2015 at 23:32
• @MichaelT Lumens measures a product of candela and steradian, hence why I avoid looking for those as I want raw power, not power over a set cone, area. Thanks for the link though. Dec 8, 2015 at 17:53

The unit you are likely going to try to work with is foot candle which is about 1 lumen/square foot (or there abouts). Lux if you want to deal with SI units and meters... but I'm on the left side of the Atlantic and so you're getting feet. Converting from fc to lux is left as an exercise to the reader. Full sunlight is about 10,000 fc. Overcast day is about 1000 fc. The moon is about 250 fc (that is the moon itself appears to be illuminated at 250 fc).

If you have a 500 lumen flashlight that spreads out over 100 square feet at the distance, that's 5 fc. If it only spreads out over 50 square feet, that's 10 fc. This is the calculation you are after. The beam will travel an unlimited distance, its just that it will spread out to the point where you can't see it.

If you are dealing with variable distances, you are going to have fun trying to redo the estimations for how long to paint each part of the scene.

At this point you figure out what the exposure you want on area in foot candles, say, you want it at 200 fc (and its the 50 square feet for 10 fc), that you are shooting at ISO 100 and need an exposure of 4 seconds for a 200 fc illumination (these are made up numbers - just to give an example of the type of calculation you will need to do), you need to shine the light on that area for 80 seconds (10 * 20 * 4).

However, this is only going to get you in the ballpark of what you are after, you will still need to do a bit of trial and error. This is because even tough its dark, there are many stops of light between one darkness and another. Light with no moon is a stop darker than light with a crescent moon, which is a stop darker than light under a half moon, which is a stop darker than light under a full moon, which is a stop darker than light under full moon with a snowscape. There are 13 more stops of light between the scene under the full moon and the exposure for the full moon (to the dismay of many people wondering why their moon photos are blown out).

The amount of contrast that you will desire depends on these factors. Too much light (and too little exposure on the rest of the scene) and what you are after is lost off one end or the other of the dynamic range.

Part of the point that I'm making here is that there are enough other variables in the 'how long to paint with light on a given area' that the amount of fudging for the lumen output isn't significant (or isn't significant to the orders of magnitude that photographers work with - 1.5x would be a big change for the manufacturer, its less than a stop for us).

For light painting, its a more a matter of 'get one that has the right color of light you are after (do you want LED 5500K light? or something more like an incandescent 2000K light?)' and then do some basic calibrations to get an idea of how much it spreads out at 10 and 80 feet away so that you can have an idea of how many foot candles you have at other distances so that you'll know if you want to light up a given area for 5 seconds, 5 minutes, or 50 minutes.

And then... try it. There is an amount of artistry that is necessary in light painting that can't be pinned down with numbers.

• Some day I hope to see the US wean itself off "squared feet" and move onto grown up SI units, :-) Dec 8, 2015 at 21:45
• @StephenG That would be nice... but its a useful measure that allows nice ballpark conversions from lumens (what you get from flashlights) to illumination. 1 lumen / m^2 is one lux, but reading about old photography tools, that's a foot candle. To someone familiar with the conversion of lumen to lux and wanting to use that instead, not an issue. That said, my old light meter - foot candles. Reading the Ansel anecdote - foot candles.
– user13451
Dec 8, 2015 at 21:55
• @michaelT When I first read foot candle, I imagined a candle that's a foot long. I'm use to SI units so 1 lu/meter^2. In any event, CRI should be added to the note as it's a measure of how "realistic" the light (how does it compare to the sun). I don't know how the compares to color. In any event, I'm waiting for my lights and I was curious if there's a good "value" or ballpack as you said. Thanks. Dec 8, 2015 at 23:59