You’ll know it, and if not, you’ll know it right now. It really matters how you convert a three channel color photo (RGB) to black and white.

I shot this one this morning on my way to work with the new 14mm Sigma lense. The sky was a little bit red toned, with bluish cloud stripes. So the only way to get details in the sky without reducing the global contrast I want here, is to exaggerate the red channel and lower the blue and green channels. So I get the subtle differences in the sky out.

I used the channel mixer from UFRaw to convert this RAW file twice with different settings. The first one is just desaturated. The second one is the result of tuned channel mixing. As a side effect the red trailer and the red hubs of the tractor
turns bright (what is fine) and the blueish damp — bluish because of the blueish dawn light — disappears a bit (what is not what I intended). So If I would present this photo somewhere I probably would combine both images or do some selective adjustments in a way to get the damp of the desaturated image into the channel mixed.

There are tons of tutorials out there targeting black and white conversion including channel mixing and other methods. There is a one from Adobe, from Cambridge in Colour and one especially for Gimp.

Of course, especially in this case, if you exaggerate the red channel, you’ll run easily in difficulties with noise and broken gradients. This is because in daylight the sensor of a DSLR typically captures relatively a lot of green, some blue and a very little amount of red tones. And stretching the red channel over the whole tone range like I did here leads to gaps in the histogram.

This is explained in this libraw article, including a possible workaround. Very interesting.