The Essential Knowledge Behind Great B&W Conversions
By John Ruttenberg
Why is good B&W conversion so hard? Most people would like to be able to push a button or follow a simple recipe. Yet all good procedures for this require some manual interference, a fine tuning step of some sort. Why?
Conversion to B&W is essentially ambiguous. What should happen to two very different colors which are roughly equally bright? Should they be converted to the same shade of gray? In not, which should be darker?
How Photoshop Decides
When Photoshop converts to gray, either esp licitly, or when generating the L channel of LAB, it uses a simple formula:
60% Green + 30% Red + 10% Blue
That's all it does. It just looks at each pixel in the color version and determines what shade of gray to use based on that formula. Conversion to LAB applies a gamma curve which lightens up the L channel a bit compared to conversion to gray scale, but the difference is only a slight difference on overall brightness, not in the relative treatment of different colors. Does this formula do the right thing? It depends on the image. Here is a contrived example. Photoshop converts the colored stripes in the first picture to the very same shade of gray as shown here. Naturally, this example was contrived to make a point. Yet, in this case, Photoshop's built in B&W conversion completely loses the contrast between the stripes. Once this conversion is made, no amount of image manipulation will restore that contrast.
The real point, though, is that there is an important set of decisions to be made. These decisions are artistically significant. They can completely change the impact of the image, determine it's emphasis. You are the only one who should be making them for your own images.
If you want to experiment with this a little, download the color image, copy each channel into it's own layer, and then play around with the opacity and blending options of the layers. You won't have any problem producing the versions I made as well as an infinite number of others. Or do the same with your own image.
There is a huge amount of lore on dgrin and beyond on this topic. In particular, see Andy's B&W conversion tutorial and this thread. Beware, when you read these that any recipe or process which doesn't emphasize the basic decision making aspect of conversion is likely to be at least somewhat misleading.
The very best thing you can do to understand how to make great B&W conversions is to buy Dan Margulis' Professional Photoshop and read the chapter Friend and Foe in Black and White. Nothing else will take you as far, and in the long run nothing else is easier because nothing else addresses the root issues as directly or as deeply.
Please address discussion of this tutorial to this dgrin thread.