A tip to achieve a better UniWB

A tip to achieve a better UniWB

Postby DPP++ » Sun Oct 14, 2012 9:29 pm

The goal of the UniWB technic is to achieve a custom white balance that will not modify the raw values, in particular the red and blue values, so as the RGB histogram that will be displayed by the camera will be as close as possible to the raw histogram.

This white balance is usually refered to by 4 values RGGB (Red, Green, Green, Blue) and these values are generally x 1024 1024 y (or x 1 1 y if the values are normalised) and the objective is to achieve a white balance that provide 1024 1024 1024 1024.

I have tried the lens cap method and the method of blowing all the pixels but it doesn't work on my classic 5D.

So I downloaded a file from this site : http://www.guillermoluijk.com/tutorial/uniwb/index_en.htm that was prepared using the magenta chart method described there. There are also lot of other files there for other cameras.

Using this file I obtained the following values : 1015 1024 1024 986

This was good but I was wondering if it is possible to achieve a better result ? And the answer is yes ! After using the method I will describe below I was able to achieve the following values : 1016 1024 1024 1027. using the same downloaded file.

So here is the idea. It's very simple. It consists in using the White Balance SHIFT feature of the camera. This feature allows to correct the white balance.

Image

So after setting the white balance to CUSTOM using the downloaded file, I did 8 shifts of the WB corresponding to the 8 surrounding points around the center of the axis that are displayed in the camera when you call the SHIFT/BRACKETING feature (use SHIFT not BRACKETING).


Image

For each shift position, I took a shot and then using exiftool I wrote the obtained values. This gave me the following table :
Code: Select all
  997:991    1005:968     1016:944
1006:1009      +          1025:961
1016:1027    1024:1003    1034:978

The + represents the center of the axis and around are the values of R and B channels (I don't write the values GG as they are always 1024).

After this first table, I saw that the results are better in the bottom left corner. So I decided to go 2 positions further from the center in that direction and took a serie of other shots and obtained a more complete table :

Code: Select all
987:1012     997:991     1005:968     1016:944
997:1030    1006:1009       +         1025:961
1006:1049    1016:1027    1024:1003    1034:978
1016:1068    1025:1045    1033:1022    1043:996

So I noticed that there is no better improvement going further.
The best position is the position that offers the smallest distance from the 1024 value in both channels. So in my case it is the position corresponding to 1016:1027.
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Re: A tip to achieve a better UniWB

Postby MzytengaM » Thu Sep 12, 2013 10:19 pm

this post is Master Full...and the link is very very usefull..
i will try one of cr2 files for white balance..
i notice my camera adding to muche blue to photos
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