Measuring lux with a camera

9 posts / 0 new
Last post
dimeotane
Offline
Last seen: 2 years 6 months ago
Joined: 01/09/2011 - 16:56
Posts: 118
Measuring lux with a camera

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      f2.8f4f5.6-one candle at one meter
5     f2.8f4f5.6f8 
10    f2.8f4f5.6f8f11 
20   f2.8f4f5.6f8f11f16-Trustfire F20 (AA Nimh) on low at 1m
40  f2.8f4f5.6f8f11f16  
80 f2.8f4f5.6f8f11f16  -Powerlight at 1m
160f2.8f4f5.6f8f11f16    
300f4f5.6f8f11f16    - my dining room lights
645f5.6f8f11f16     -Trustfire F20 (AA Nimh) on high at 1m
1290 f8f11f16       

 

Sources:  you can confirm my findings here: please share your comments on this method:

http://www.natmus.dk/cons/tp/lightmtr/luxmtr1.htm formula:
Lux = 50 x fnumber^2/ (exposure time in seconds x ISO film speed)

http://www.digicamera.com/features/goinggoinggone/ formula:
Lux = 70 x fnumber^2/ (exposure time in seconds x ISO film speed).

http://www.firstrays.com/measurelight.htm - online calc

Edited by: dimeotane on 03/17/2011 - 11:53
Don
Don's picture
Offline
Last seen: 4 months 1 week ago
Joined: 01/12/2010 - 16:32
Posts: 6617
Location: Scotland

Interesting, thanks for this.

 

The numbers from my light tests are always to be found here.

https://spreadsheets.google.com/ccc?key=0ApkFM37n_QnRdDU5MDNzOURjYllmZHI...

Haggai
Offline
Last seen: 5 days 6 hours ago
Joined: 01/06/2011 - 04:05
Posts: 1012
Location: Israel

I took the bait, and measured my lights.

I used F5.6, ISO200 for all measurements, and converted it using the 50X formula from natmus.

 

emittermode     spot      spill
ambient 22
DX C30 P4flood235157
 throw501813
iTP R5low104
 med6310
 high15724
tMart Q5 Ilow1575
 med125439
 high250978
tMart Q5 IIlow2358
 med98031
 high250978
Manafont XML   low23510
 med125478
 high3136118

The measurements make sense, but are not too impressive... Undecided

Don
Don's picture
Offline
Last seen: 4 months 1 week ago
Joined: 01/12/2010 - 16:32
Posts: 6617
Location: Scotland

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. Wink

 

The numbers from my light tests are always to be found here.

https://spreadsheets.google.com/ccc?key=0ApkFM37n_QnRdDU5MDNzOURjYllmZHI...

angusinalberta
Offline
Last seen: 9 years 10 months ago
Joined: 01/19/2011 - 12:20
Posts: 37
Location: Canada

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.

[1] http://chdk.wikia.com/wiki/CHDK

Cheers,
Angus

 

 

 

angusinalberta
Offline
Last seen: 9 years 10 months ago
Joined: 01/19/2011 - 12:20
Posts: 37
Location: Canada

graph 

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.

 

[1] http://chdk.wikia.com/wiki/CHDK

Cheers,
Angus 

ankaka
ankaka's picture
Offline
Last seen: 8 years 9 months ago
Joined: 04/12/2012 - 01:22
Posts: 6
Location: no

Thanks for sharing in the above information.

M2, M3, Rolleiflex 3.5 B, Yashica Mat 124G, Nikon D90, Nikon F100, Nikon FM, Pentax K1000..   Advice and constructive criticism always welcome..

http://www.sencart.com/Supply-camera_c76

wle
wle's picture
Offline
Last seen: 11 hours 19 min ago
Joined: 01/07/2015 - 13:49
Posts: 2131
Location: atlanta ga

isn;t 1 candle at 1 meter, 1 lux, though – not 2?

i don;t get it

wle

"You never have the wind with you - it's either against you, or you're having a good day."
    Daniel Behrman, "The Man Who Loved Bicycles".
It never gets easy, you just go faster.   
-Greg Lemond.
       ,ø¤º°`°º¤ø¸,ø¤º°`°º¤ø¸,ø¤º°`°º¤ø¸

dchomak
Offline
Last seen: 2 years 8 months ago
Joined: 03/17/2012 - 20:10
Posts: 4122
Location: Connecticut

Chinese lux would be 2, or sometimes 3 or more Wink