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.
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.
|0, 94, 255|
|167, 0, 0|
|21, 76, 0|
|0, 237, 255|
There's no pattern here. Next, we'll look at theirvalues. HSB stands for hue, saturation and brightness. Perhaps the color's brightness determines it's grayscale value.
|218°, 100%, 100%|
|0°, 100%, 65%|
|104°, 100%, 30%|
|184°, 100%, 100%|
Again, there is no pattern. What about the colors'values?
|25, -127, -127|
|25, 127, 127|
|25, -50, 50|
|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.