In this method, we simply click Image > Mode > Grayscale in Adobe Photoshop
and our image is converted to black and white. The advantages of this
method are that it is quick, intuitive and retains perceived luminance.
There is no single algorithm Photoshop uses to calculate
grayscale. It is based on the setting specified for Gray in the
Working Spaces parameters of the Photoshop Edit > Color Settings dialog box.
However, the disadvantages are many. This method is a
destructive method. The image is literally no longer a RGB image.
Instead of three color channels, there is now only one grayscale channel.
The specific information captured in the three color channels has been
permanently lost. Some advocates say we can reconvert the image back to
RGB. However, we do not get back the original RGB channels. We get
the grayscale channel replicated into the red, green and blue channels.
This does not restore the original RGB information. Therefore, we lose the
ability to manage tone by color. To demonstrate this to yourself, convert
an image from RGB to true grayscale and back to RGB (click Image > Mode >
Grayscale and then click Image > Mode > RGB Color). Did any of the colors
reappear? No, because all color information was lost when first converted
Figure 1 shows the two sample images converted to true grayscale. Figure 2
shows the same two images converted in such a manner as to retain
measured luminance. Which is more accurate? The choice is yours.
Grayscale retains the perceived tone as defined by %K. The Measured
luminance approach retains the measured tone as defined by the Curves control.
Figure 1. Grayscale, or Perceived Tone
Figure 2. Curves, or Measured Tone
If you prefer grayscale for your black and white images,
I do not recommend using this method. Instead, I recommend using
the technique described on the
System Grayscale page.