RAW vs White balance

Started Nov 20, 2006 | Discussions thread
edhannon Senior Member • Posts: 1,761
Re: Manual WB does impact *istD RAW

JonasB wrote:

When plotting the data, where from do you get the numbers?

I wanted to use the RAW histogram program referenced above on my *istD. However, the *istD does not pack the 12-bit RAW data into two 16-bit words like the latter ones do. However, I do not have a Java compiler. So I could not just modify the souce code provided.

So I wrote a quick basic for applications (QBA) module in Excel that adapted the algorithm used in the Java source referenced above.

A side benefit of doing it in Excel QBA is that I ended up with the RAW histogram bin counts from the file in a spreadsheet. These are the counts of how many pixels have a specific value. There are 4096 bins, one for each of the possible values of the 12-bit RAW data.

This allows me to do statistical analysis on the RAW data before it is processed through any converter.

My first question was how many stops above the meter reading does the first channel saturate? To answer this I modified the QBA to read in multiple files and place the histogram bin data in sets of three columns.

I also compute several statistics for each file: average value , standard deviation, median value, how many pixels are 0 (pure black) and how many are 4095 (pure white)

I then set up a curve in Excel that plots the median value for each of the curves against the EV computed from the exposure and apperture.

Then I shoot a series of "white wall" images with exposures separated by 1/2 stops, run them through my Excel QBA and then sort them by EV.

This results in a tone curve for each of the three colors.

The curves I posted show that the first channel clips about 3-stops above the meter reading - regardless of the WB setting. I used a Minolta Spot Meter F that I have calibrated against my *istD and a 77mm limited to give a vaule of 90 when converted with default settings. This correlates well with the internal meter in daylight - but not in tungsten - that is an issue for a different post!

They also show that the *istD does not change the RAW values with preset WB settings but that it does with manual WB.

Ed Hannon

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