| Article Index |
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| 1. Test protocol |
| 2. Data analysis |
| 3. Calculation: |
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This article explains how to quickly and easily measure in a reliable and repeatable way the RGB coefficients of your DSLR. I use the Iris software to calculate the data. This is a translation of the french article. Only the test is translated, not the explanations... Keywords : RGB coefficients, DSLR, white balance. |
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1. Test protocol
All pictures shall be stored in the RAW format. Be sure to select the right DSLR in Iris in order to get a correct parsing of the red/green/blue channels of the picture.
When:
- a sunny day when the sun is about at 45° elevation.
With what:
- a Canson white 180 g/m2 type "C" paper sheet (you can also use a photocopy paper, or any other white area such as house walls, windows plastic fabric but the results will not be repeatable) ;
- the DSLR to test, set between 400 and 1600 ISO, with a 150 to 200 mm lens, speed faster than 1:1000 set so that the picture of the white paper is about 50% of the dynamic (bracket with the aperture and the speed to have at least one acceptable picture).
How:
- install the paper sheet in front of the sun, away from colored or reflective areas.
- shoot some pictures of the white paper, taking care not to focus on the paper (set focus to infinity) and not to include any umbra in the picture frame. Bracket with some speed/aperture but speed shall be faster than 1:1000 s.
Then:
- shoot one picture at the fastest speed in the complete dark, at the same ISO setting. This will be the offset frame.
2. Data analysis
1. Start Iris
2. Click on the camera icon (below the Processing-Spectro menus) and uncheck the "Apply" box in the White balance area. Select the right DSLR (if not listed, you will have to test several other DSLR of the same maker).
This is a french version of Iris but the screens are organised the same way in the english version.
3. Load the Offset picture from the menu File > Load... then save it in a .PIC format (say offset.pic) , now load one of the white sheet picture
4. Click on the Command icon located just on the left of the camera icon of the step 2
5. In the Command box, type in STAT after the > sign (do not erase the >) then type Enter.
6. In the Output window, check that the means are all between about 500 and 2000 ADU, otherwise choose another white sheet picture.
7. In the Command window, type SUB offset 0 then press Enter
8. In the Command window, type STAT again and press Enter
9. In the Output window, mark out the mean values of the Red, Green and Blue channels
3. Calculation:
- Rcoef = G / R
- Gcoef = 1.0
- Bcoef = G / B
For example, let say we found:
- R = 377.9
- G = 783.6
- B = 545.2
The 3 coefs will be :
R = 2,07 V = 1,00 et B = 1,44
It will take about 20 minutes to do the exercise.