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03-02-2010, 06:55 PM   #1
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K-x as a light meter

Hey everyone,

I was wondering if anyone ever tried to use their DSLR camera (the Pentax K-x for my case) as a light meter to test the luminance of a light source. and if so, what are the methods used?

03-02-2010, 10:00 PM   #2
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It wouldn't work terribly well. First off, true 0 EV is 1 second exposure at aperture F1.0. From that, you can probably extrapolate what the light reading is based on your camera's current shutter speed, aperture and ISO settings, but good luck doing all that math. You also need to take into account what the camera's meter says and add that to your calculations.

Maybe if someone made a chart with all the different values at specific settings, it would be possible. But figuring it out on-the-fly sounds like a nightmare.
03-02-2010, 10:23 PM   #3
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I do use a dslr as a flash meter when I am combining a bunch of flashes for a shot on film.
10-14-2010, 05:01 PM   #4
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I was trying to figure out if we could do that with the K-x as well, after reading the understanding exposure book. But I have no idea how.

10-14-2010, 05:54 PM   #5
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hmm, well i do use my k-x as a "lightmeter" for my Honeywell Spotmatic with a non-functional lightmeter
10-14-2010, 11:04 PM   #6
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QuoteOriginally posted by RolloR Quote
hmm, well i do use my k-x as a "lightmeter" for my Honeywell Spotmatic with a non-functional lightmeter
I used to employ my 1.1mpx Sony DSC-P20 p&s to meter for my Voigtlander Vito II and Zeiss Ikonta B 521/16 folders. Then I got a Sekonic handheld meter, which is rather faster/easier to use -- no need to wait for the power-up, and that little Sony LCD screen is damn hard to read in daylight.
10-15-2010, 03:44 AM   #7
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I use my K20D as a de facto light meter. I do this afterwards using the EXIF info to work out the Light Value (LV) that produced the exposure, but to some extent, you can so it in the field too.

First off, consider the "Sunny 16" rule:

The basic rule is:

"On a sunny day set aperture to f/16 and shutter speed to the [reciprocal of the] ISO film speed [or ISO setting]."

Sunny 16 rule - Wikipedia, the free encyclopedia

For example:
1/100s, f/16, ISO100
1/125s, f/16, ISO125
1/200s, f/16, ISO200

A Light Value calculation shows that these EXIF values indicate a 14.6 LV level (a sunny day). See here for my LV calculator.

A link explaining EV (Exposure Value) is here. Look at Table 2.

EV is related to LV, but EV is for ISO100, aka EV100, whereas LV incorporates ISO changes.

1 EV = 1 LV = 1 Stop.

At the beach I've found the light maxs out at 15.6 LV. Using a f/16 aperture (I don't usually stop done this far due to diffraction softening, but I'm showing it here to relate it to the Sunny-16 example), some examples of the shutter speed & ISO to reach 15.6 LV are:

1/200s, f/16, ISO100
1/400s, f/16, ISO200
1/800s, f/16, ISO400

or in the example in the mentioned Wikipedia link:

1/100s, f/22, ISO100, since f/16-> f/22 is 1 stop less aperture area/less exposure. So to produce the same exposure level on the sensor or film, the light level must have been 1 stop/EV/LV higher.

So you could say 15.6 LV brightness requires the application of the "Very Sunny 22" rule.

Say, you're the sort of person who likes to walk around with the manual settings of
1/125s, f/8, ISO100. That's 13 LV (cloudy bright - no shadows)

While 1/125s, f/5.6, ISO100 is 12 LV (heavily overcast or in the shade on a sunny day), because, f/8 -> f/5.6 is 1 stop greater aperture area.

So, if you can remember these two:
1/125s, f/8, ISO100 = 13 LV
1/100s, f/16, ISO100 = 14.6 LV

it's not hard to work out ball park LVs in the field using just mental arithmetic, at least with full-stop steps in ISO, shutter speed & ISO sensitivity. With .33 or .5 stop steps it becomes more difficult and that's where a LV calculator comes in handy.

Note: the camera-as-LV-meter method assumes that the shot is well exposed.

Dan.

Last edited by dosdan; 10-15-2010 at 05:14 AM.
10-15-2010, 05:32 AM   #8
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I think the most reasonable way to use the camera as a light meter is to calibrate it with a subject of known luminance - perhaps using a light meter you trust as a standard.

Dave

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