Adobe Photoshop CS5 for Photographers - Martin Evening [169]
The dumb black and white conversions
When you change a color image from RGB to Grayscale mode in Photoshop, the tonal values of the three RGB channels are averaged out to produce a smooth continuous tone grayscale. The formula for this conversion consists of blending 60% of the Green channel with 30% of the Red and 10% of the Blue. The rigidity of this color to mono conversion limits the scope for obtaining the best grayscale conversion from a scanned color original (see Figure 6.1). The same thing is true if you make a Lab mode conversion, delete the a and b channels and convert the image to grayscale mode, or if you were to simply desaturate the image. There is nothing necessarily wrong with any of these methods, but none of these are really making full use of the information that can be contained in a color image.
Figure 6.1 If you convert a color image to grayscale mode, Photoshop pops the dialog shown here which is basically advising you there are better ways to convert to black and white.
Smarter black and white conversions
If you capture in color, the RGB master image contains three different grayscale versions of the original scene and these can be blended together in different ways. One of the best ways to do this is to use the Black & White image adjustment in Photoshop, which while not perfect, can still do a good job in providing you with most of the controls you need to make full use of the RGB channel data when applying a conversion. The Black & White slider adjustments will, for the most part, manage to preserve the image luminance range without clipping the shadows or highlights. These adjustments can be applied to color images directly, or by using the Adjustments panel method to add an adjustment layer. The advantage of using an adjustment layer to apply a black and white conversion is that you can quickly convert an image to black and have the option to play with the blending modes to refine the appearance of the black and white outcome. Let's start though by looking at the typical steps used when working with the Black & White adjustment controls.
1.
The following steps show a basic method for converting a full color original photograph to black and white. The Black & White image adjustment can be applied directly by going to the Image Image Adjustments menu, or you can go to the Adjustments panel and click on the Black & White button (circled) to add a new adjustment layer.
2.
To begin with I clicked on the Auto button. This applied an Auto slider setting based on an analysis of the image color content. The Auto setting usually offers a good starting point for most color to black and white conversions and won't do anything too dramatic to the image, but is immediately a lot better than choosing Image Mode Grayscale.
3.
If you don't like the auto setting result, you can adjust the sliders manually to achieve a better conversion. In this example, I lightened the Reds and Yellows and darkened the Cyans.
4.
Lastly, I clicked on the target adjustment mode button (circled) for the Black & White adjustment. This allowed me to move the cursor over particular areas of interest (such as the sky) and dragged directly on the image to modify the Black & White adjustment. This step selected the nearest color slider in the Black & White adjustment panel. Dragging to the left made the black and white tones go darker and dragging to the right, lighter. The end result was a photograph in which I managed to lighten the sculpture more and increase the contrast in the sky.
Black & White adjustment presets
As with other image adjustments, the Black & White adjustment has a presets menu at the top from where you can select a number of shipped preset settings. Figure 6.3 shows examples of the different outcomes that can