The importance of -b-ing during conversion (or how to set the blacklevel correctly)

In a RAW file, even when takinblacklevel_unsetg an image against the lens hood the values are not zero. Black is actually quite a bit above zero. The raw converters base their white balance calculations on the black level, that is the value that should be zero. As the green channel in the raw file is more sensitive to light than the red or blue channel, those have to be increased in postprocessing to get correct colors. If this calculation is based on the wrong starting point you get something like this image on the right.

The horse (being black in reality) looks rather magenta-ish, even though the brighter areas of the image look good. Setting the white balance on the horse’s fur doesn’t help as those brighter areas then become as green like an underwater shot in a pit lake.

blacklevel_setOnce you call the conversion using the parameter -b things do look a lot better. The image on the left has been developed using exactly the same parameters in Lightroom as the one above, the only difference being the use of the blacklevel switch.

This switch sets the blacklevel to 92 if you don’t pass any other number directly after -b. This is the value that works for my M8 and consequently I set that as default.

If your image still has a slight color cast in the darkest shadows you can change that by adapting the blacklevel value. For fixing a magenta color cast, increase the value (i.e. m8raw2dng -b 97 ...), a green color cast can be removed by decreasing the value (i.e. m8raw2dng -b 86 ...). You’ll have to experiment a little bit to find the correct value for your camera. Once the correct value is found you can use it on all your images.