Suppose I have a light and I want to determine how many lumens it produces. Is there some way to use an SLR camera’s (Canon 7D) built-in light meter and use the resulting ISO, f-stop, and shutter speed to do some kind of units conversion to lumens?
I know from past experience that converting photometric units and radiometric units is an incredibly complex and frustrating task. It would be a bit easier if your Canon 7D directly gave you a reading in some type of unit, rather than giving you an ISO speed or f/stop or shutter speed. But not much.
A Lumen is the total quantity of light emitted by a body in all directions, weighted by the response of the eye. It’s luminous energy per unit time. For giving you the photographic settings, your camera is probably using illuminance – the luminous flux per unit area, measured in lux. Lux is a lumen per square meter*. To go from lux to lumens you have to make some assumptions about what proportion of the total irradiated area your sample represents. The easiest is to assume the total flux goes equally into all directions, so that you can assume a sphere of surface area 4(pi)r^2, where r is the distance you are from your subject. It’s probably not really correct, but it’ll get you a ballpark answer. All you have to do then is find out the relationship between whatever number it gives you (shutter speed or f/stop or ISO), which you can maybe get from your manual.
It’s probably easier to just get a light meter and measure things, or to borrow one and see how its reading correlate with what your camera tells you.
*Photometric and radiometric units come in a bizarre variety of name – phots, lux, lumens, stilbs, candles, candelas. Don’t say them too fast or you’ll have to wash your mouth out with soap.
Have a look at the table of luminous units here. Be very careful as to what you are measuring total luminous flx, total radiometric flux, flux per unit area of source, flux per unit solid angle, etc.
OK, so maybe not. Just FYI I am trying to compare flashlight brightness of what I have vs. what I might buy.
Since one lux is one lumen per square meter, back away until the beam is covering a square meter (appx.) and get the lux reading.
His camera doesn’t give readings in lux (or lumens) – it tells him what ISO film speed he should be using, or what shutter speed, or what f/stop.
:smacks forehead hard:
There are light meter apps. I use this one.
Physics Toolbox does it, too (assuming your phone has a light intensity sensor.)
Googling, I see someone even wrote a paper using it:
https://iopscience.iop.org/article/10.1088/1742-6596/1467/1/012056/pdf
Aside: @Darren_Garrison , if you’re also the one who recommended that app to me many moons back, thank you.
I think it may have been. I know I’ve talked about it here before.
I found this table which converts between EV (exposure value ) at ISO 100
EV Lux
-1 1.25
-0.5 1.75
0 2.50
0.5 3.50
1 5.00
1.5 7.00
2 10.00
2.5 14.00
3 20.00
3.5 28.00
4 40.00
You can convert between EV and aperture/shutter speed with the formula:
EV = log₂ (N²/t)
where N = f-stop and t = shutter speed.
Be careful about comparing lumen claims.
Yes, I am reviewing several brands and they are all over the place. One model claims 2300 lumens which just has no credibility. Most I’ve seen are in the 400-1000 range, at least as far as claims. Your link is the first one I’ve seen that actually tested any. I tried Consumer Reports but they haven’t rated flashlights.
Since your end game regarding lumens is choosing a ‘good’ flashlight can I add some non-technical points? Based on my modest experience with a few reputable, mainly Chinese LED flashlights.
A simple AAA battery keychain flashlight (barely the size of most people’s little finger) can push out a genuine 100 lumens. In practical terms this will light your way inside a house or along a path outside.
Many torches have low power modes down to 1 lumen (or less) and this light level is enough to discretely navigate inside a familiar building without disturbing others. However the point is, speaking visually (and not necessarily scientifically), 1 lumen doesn’t ‘look’ one hundredth the brightness of 100 lumens. Indeed the basic truism is you need to multiply the lumens by about 4X to get ‘one step’ brighter.
Hence the next step up from a 100 lumens is 400-500 lumens which is generally agreed to be quite bright enough for pretty much anything short of air/sea rescue. That said many flashlights aim for 1,000 lumens. Probably because it is a round number. A 14500 battery (same size as AA) flashlight of decent quality should be able to manage a genuine 600-700 lumens although not for very long. But from a flashlight you can slip into the smallest purse or shirt pocket.
There are many flashlights that genuinely reach 2,000 to 5,000 lumens while still being quite small but such flashlights tend to get quite hot at full power and will either step down (reduce brightness), or get too hot to hold or indeed damage themselves (burn out.) Note any unfamiliar brand name claiming to achieve 50,000 lumens from a single small battery is obviously lying.
The brightness in lumens is only one factor with a flashlight. The type of batteries required, the running time from a fresh set of batteries plus the features (how many power settings for example) all count. Size, cost, cosmetics and ergonomics.
However lumens is not the only PRACTICAL issue here.
The physical design of the flashlight can be chosen to emphasise the beam quality. The two qualities here are ‘floody’ or ‘throwie’ where a ‘floody’ design sends the light out in a wide pattern (lighting around your feet) while a ‘thrower’ (which typically have a more bulbous head) concentrates the beam and sends it further. Hence two flashlights, both 1,000 lumens, the ‘thrower’ will illuminate FURTHER away but the ‘floodie’ will illuminate wider which for some is more useful. That said it’s easy to fit a diffuser on a thrower but not practical to try a focus a floodie.
Then there is the colour quality of the beam. The three main types of white beam to consider are Cold White, Warm White and High CRI. Cold White beams have a high colour temperature (above 5,000 degrees K) and although ‘white’ they have a blue-ish quality. Cold White LEDS (usually) have the highest lumens so look and indeed are brightest… But in general use (inside a home, reading a book) some people find the light unpleasantly harsh after a while. Admittedly others don’t care.
Warm White (below 4,500 K) have a yellower tint which many people prefer. Also very, very warm (3000 K) not only looks ‘yellow’ (as opposed to merely yellow-ish) it is considered to have much better penetration in mist and fog. If you often need a flashlight in mist (live near the coast) then a warm tint might be better than more lumens in a colder tint.
Something at around 4,500 to 5,000 K would be Neutral White but High CRI while usually neutral, is something slightly different. When LEDs are used to create white light they typically have an incomplete spectrum. In simple terms they look white but they are unable to accurately reproduce ALL colours. Accurate reds are the usual weakness but it could be any colour. A High CRI LED (usually meaning 90 CRI or the better 95 CRI) will make a better job of colour reproduction. Hence if colour fidelity is important a High CRI option should be chosen over a simple high lumens.
Examples of when a HIGH CRI is useful are: Looking at nature, plants and animals at night, working on colour coded electronics and the need to differentiate between red, maroon, purple and brown fuses and wires, studying paintings and other artworks or making colour sensitive repairs.
Final point. I own a few very low CRI flashlights and a couple of HIGH CRI flashlights and have to admit that in general use even a pathetically low CRI isn’t really a problem. For me that is.
TL:DR For most non-professional uses a genuine 500-1,000 lumens is all you will really need. When choosing the right flashlight for you there are a lot of other factors to consider.
TCMF-2L