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----------------------- Sensor Analysis Primer - BlackLevel Study

--------------------------------------------------------- By Bill Claff

Introduction

In raw sensor data some value in Digital Numbers (DNs) represents zero or no light; this is black level.

It's important to know black level to properly process raw data.
This value is stored in the Exif data of the raw file in the BlackLevel tag.
There are also some specialized tags for BlackLevel on a row by row and column by column basis.

Analysis

I studied black frames for nearly 300 cameras at all ISO settings and compared the observed black level to the values in the Exif.
When multiple BlackLevel values were present I used the average across channels, rows, and columns.

Note that BlackLevel is zero for about 25% of the cameras studied.
Since this affects the observed average (Signal) and standard deviation (Noise) I separated results based on Zero or Non-zero BlackLevel.
There were also a very small number of outliers I excluded from the chart for clarity; this is less than 0.4% of the data.

In the analysis I use a z-score which is how many standard deviation the observed value is from the Exif value.

Here are the results:

Conclusion

BlackLevel values appear to be quite well behaved.
Even for Zero BlackLevel cameras the observed black level is seldom even one standard deviation away.
Unless we're operating at signal level quite close to zero imprecision in the BlackLevel value is not an issue.
Long exposures (such as for astro-photography) with significant dark current were not studied; they may very well exhibit more variation.