Most of us don't have a lux meter to measure the lights we buy and review. Giving an accurate review of a new light is difficult unless you can describe approximately how bright the light is.
But it is possible to use your camera to measure an approximate brightness in Lux. This method I've shown below is often used by people measuring light for plants, art galleries and other things where it is important to measure how bright the illumination is.
A basic understanding of using a camera to set ISO, aperture and shutter speed combinations is required on manual mode. Make sure your camera meter is 'averaging' and not spot metering.
You can figure out the LUX illumination falling on a standard letter sized white sheet of paper using the built in meter of a still camera by placing a sheet of white paper perpendicular to the light source at 1 meter away and set the camera light meter to ISO 100. Fill the viewfinder with the page. You can now take a reading and convert the meter’s reading to LUX.
You may wish to start with measuring a 'standard' full candle flame at 100cm to confirm a correct reading of 2 lux.
This chart is approximate in each level being double the illuminance. In photography changing standard shutter speed or aperture settings 'one stop' exposes double the amount of light (or half). But many high end cameras will allow you take 'half stop' readings for a greater amount of accuracy.
Lux
@1/30
@1/15
@1/8
@1/4
@1/2
@1s
@2s
@4s
@8s
example light readings:
2
f2.8
f4
f5.6
-one candle at one meter
5
f2.8
f4
f5.6
f8
10
f2.8
f4
f5.6
f8
f11
20
f2.8
f4
f5.6
f8
f11
f16
-Trustfire F20 (AA Nimh) on low at 1m
40
f2.8
f4
f5.6
f8
f11
f16
80
f2.8
f4
f5.6
f8
f11
f16
-Powerlight at 1m
160
f2.8
f4
f5.6
f8
f11
f16
300
f4
f5.6
f8
f11
f16
- my dining room lights
645
f5.6
f8
f11
f16
-Trustfire F20 (AA Nimh) on high at 1m
1290
f8
f11
f16
Sources: you can confirm my findings here: please share your comments on this method:
I must try this with my camera and see how the numbers correlate with the lightbox. The meter in the camera is probably a lot better than the DX lightmeter - and I also own a very good Sekonic lightmeter. I must get around to this soon. Problem is I'm fresh out of round tuits. ;)
First off I can't understand how anyone truly interested in flashlights doesn't have a light meter. It's like an electrician without a voltmeter. Perfectly good digital lux meters are under $20 for pete's sake.
Rant over. Now back to another way to measure light with a camera.
Cameras always need to be able to measure the scene luminace in order to set the proper shutter speed and aperature.
Normally that value is hidden from the user. However if your camera is made by Canon and if you have loaded a program called CHDK [1] you can activate a nice light meter.
Do the following from the CHDK menu on your Canon.
OSD parameters -> Miscellaneous Values -> Show Scene luminance (cd/m2) and click the radio button
That puts a 'B:' followed by the luminance value, into the viewfinder.
But wait, we're still not done. The value you see is just a fraction of the lux hitting the target, depending on the reflective properties of the target. So you still need to borrow a lux meter to calibrate it.
And one more thing. The lux value you get from a camera, whichever method you choose, is a weighted AVERAGE that depends on the camera's settings. On my camera I have a choice of 'Evaluative', 'Center Weighted Avg' or 'Spot'.
A camera is quite useful to get an approximate lux value, but is no substitute for a proper light meter.
For those who might be interested in using CHDK [1] with your Canon for measuring light values, here is a typical calibration graph. For example if the value of B from the camera is 27 cd/m2 then the corresponding value is 100 lux.
Note that the graph is linear and the slope is 3.69 in this particular case. The slope depends on the colour and roughness of the wall. But the calibration needs only to be done once.
And it must be restated that the value represents the average within the frame. If you want the just the lux of the hotspot of the beam you must zoom in quite tightly and/or move the camera closer to the wall.
