Wednesday, May 23, 2007
The Ultimate Black & White
New technology and techniques are giving rise to the ability to create the best black-and-white images ever
Part I: Converting A Color Image To Black-And-White
Ironically, shooting with digital cameras arguably provides photographers with better tools than color film photographers ever had for producing black-and-white imagery. I never had much success converting traditional color film photography to black-and-white, but digital is a different story. Just about everything you need to convert to black-and-white is already there when you capture digitally.
A standard RGB digital image is composed of red, green and blue channels that, interestingly, represent what amounts to tricolor separation filters, such as the Kodak Wratten designations of Wratten 29 (red),
Wratten 47 (blue) and Wratten 61 (green) (Figure 1). Thus, in a digital image, one already has a full-color representation of a photographic scene. The trick is to determine how to convert from color to black-and-white or, in other photographic terms, how to make a specific black-and-white panchromatic response from a full-color image.
The red channel represents a Wratten 29 separation filter (Figure 2). The green channel represents a Wratten 61 separation filter (Figure 3). The blue channel represents a Wratten 47 separation filter (Figure 4).
When you take a color image and do a mode change to Grayscale via Photoshop, there's a specific panchromatic response that takes place when the conversion is made (Figure 5).
Photoshop's default Grayscale mode change performs a calculation that's roughly equal to 30% of the red channel, 60% of the green channel and 10% of the blue channel. The exact formula is dependent on the working spaces of RGB and Grayscale, and there seems to be some additional tweaking involved (Figure 6).
There are other methods available in Photoshop, however, to convert color to black-and-white.
Using the Channel Mixer adjustment and plugging in 30, 60, 10 while in the Monochrome mode will give you one flavor of converting from color to black-and-white (Figure 7).
By varying the percentages in the Channel Mixer, one can adjust the relative amounts of the various color channels when making a conversion (Figure 8).
What is not intuitive, but useful, is that you can use negative numbers in combination with positive numbers when plugging in the conversion parameters. In general, you want the sum total to equal 100% so no additional tone adjustments occur (Figure 9).
Another way of making the conversion is to use the Hue/Saturation adjustment and lower the saturation to zero (Figure 10).
Yet another variation is to convert the image from RGB to Lab and discard the A and B channels. This results in a grayscale rendering based on the luminance of the original RGB color image (Figure 11).