Black and White Conversion Theory

Why some photographs convert better than others.

Take a look at this photo.

The statue is greenish, the building is red, and the windows have a blue tint. In color, the statue stands out from the background. But watch what happens when the photograph is converted to black and white.

Everything gets converted to a similar shade of gray. The statue blends into the background. In fact, the only things that stand out are the white trim.

Take a look at this four color chart.

These four colors seem very different. They should convert to four distinct shades of gray. But watch what happens when it's converted to grayscale using Straight Grayscale Conversion.

To see why it does this we need to look at how these colors are encoded. We need to find out what the first three colors have in common and what's different about the fourth color. First, let's look at their RGB values.

Color	RGB Values
Top Left	0, 94, 255
Top Right	167, 0, 0
Bottom Left	21, 76, 0
Bottom Right	0, 237, 255

There's no pattern here. Next, we'll look at their HSB values. HSB stands for hue, saturation and brightness. Perhaps the color's brightness determines it's grayscale value.

Color	HSB Values
Top Left	218°, 100%, 100%
Top Right	0°, 100%, 65%
Bottom Left	104°, 100%, 30%
Bottom Right	184°, 100%, 100%

Again, there is no pattern. What about the colors' Lab values?

Color	Lab Values
Top Left	25, -127, -127
Top Right	25, 127, 127
Bottom Left	25, -50, 50
Bottom Right	75, -127, -127

We finally see a pattern. The first three colors have a luminosity of 25 while the last color has a luminosity of 75. The color's luminosity channel seems to determine the color's grayscale. In fact, according to "Professional Photoshop" by Dan Margulis, the color's luminosity is the only thing that determines its grayscale value. This is why some photographs convert to black and white better than others. A photograph with a greater range of luminosities will convert to a greater range of gray tones. A photograph with a narrow range of luminosities will convert to a narrow range of gray tones.

So, does this mean that photographs with a narrow range of luminosities can't be converted? Certainly not. However, it does mean that these photographs need a different conversion method than the Luminosity Based conversion methods. What you need to do is alter the colors in the photograph to change their luminosity. In black and white film photography this was done by using color filters. For example, a photographer might attach a red filter to his lens to darken the sky. In digital photography this is done by using Color Altering conversion methods.

Black and White Conversion Theory

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