- Sun May 26, 2013 3:41 pm
#177411
It's easy to calculate light intensity using only an old single lens reflex (SLR) type camera with a built in light meter and the online calculator at this link:
http://www.firstrays.com/measurelight.htm
You only need three things: your camera, a piece of white paper, and the calculator.
Simply put the paper in the location you wish to measure, facing it as close to perpendicular to the light source as possible, and point the camera at it so it fills the frame completely - trying to shoot as close to perpendicular to the paper as possible without creating a shadow. Adjust the aperture so that you get a decent shutter speed, and then use the ISO setting and those values in the calculator to determine the foot-candles of light intensity. (Note that these numbers are not exact, but certainly give a "ballpark" figure to work with.)
For example:
ISO = 100
Shutter Speed = 1/1000 second
(Ignore all the extra periods between columns, I just needed them as spacers.)
f stop......fc.......lux......lumens/sq. ft.
f/1.4......125....1345........125
f/2........250.....2690........250
f/2.8 .....500.....5380........500
f/4.......1000....10760......1000
f/5.6.....2000....21520......2000
f/8.......4000....43040......4000
f/11......8000....86080......8000
f/16....16000...172160.....16000
I made a graph of the data so I can read intensity right from the graph without having to be online when I made a measurement.
I used this a lot before I bought an actual light meter.
I posted this in my blog also.
http://www.firstrays.com/measurelight.htm
You only need three things: your camera, a piece of white paper, and the calculator.
Simply put the paper in the location you wish to measure, facing it as close to perpendicular to the light source as possible, and point the camera at it so it fills the frame completely - trying to shoot as close to perpendicular to the paper as possible without creating a shadow. Adjust the aperture so that you get a decent shutter speed, and then use the ISO setting and those values in the calculator to determine the foot-candles of light intensity. (Note that these numbers are not exact, but certainly give a "ballpark" figure to work with.)
For example:
ISO = 100
Shutter Speed = 1/1000 second
(Ignore all the extra periods between columns, I just needed them as spacers.)
f stop......fc.......lux......lumens/sq. ft.
f/1.4......125....1345........125
f/2........250.....2690........250
f/2.8 .....500.....5380........500
f/4.......1000....10760......1000
f/5.6.....2000....21520......2000
f/8.......4000....43040......4000
f/11......8000....86080......8000
f/16....16000...172160.....16000
I made a graph of the data so I can read intensity right from the graph without having to be online when I made a measurement.
I used this a lot before I bought an actual light meter.
I posted this in my blog also.
Last edited by Leilani Kimiko on Wed May 29, 2013 7:47 pm, edited 1 time in total.