There is a big difference between Desaturation and Black and White filtering. Both of them produce a grayscades image, but the algorithms are quite different. When we deal with color images, each color is defined by tree components: Red (R), Green (G) and Blue (B) (RGB model). According to this model, each color is defined by tree numbers from 0 till 255 (in byte coding), for example, red color = (255, 0, 0), yellow = (255, 255, 0) (combination of red and green), white = (255, 255, 255) and black = (0, 0, 0). After filtering we get only only one grey comonent - C, which is a combination of color components: C = [i]func[/i](R, G, B).
Black ans white filtering uses the linear combibation of all the color channels: C = a*R + b*G + c*B. If the "Preserve luminocity is switched ON, then the equality a + b + c = 1 is kept. IBy default C = (R + G + B) / 3. Desaturaton uses other algorithm: C = (MAX(R,G,B) + MIN(R,G,B)) / 2. It means that with desaturation, for eample, red and yellow colors will be represented by the same greyvalue. Indeed, C(255, 0, 0) = (MAX(255,0,0) + MIN(255,0,0)) / 2. = (255 + 0) / 2 = 127; C(255, 255, 0) = (MAX(255,255,0) + MIN(255,255,0)) / 2. = (255 + 0) / 2 = 127. With Black and White we get different result: C(255, 0, 0) = (255 + 0 + 0) / 3 = 255 / 3 = 85; C(255, 255, 0) = (255 + 255 + 0) / 3 = 450 / 3 = 150. That's why you can create additional contrast, using Black and White filter.
Let us consider an example:
[attachment=2]spectrum.jpg[/attachment]
[attachment=1]spectrum1.jpg[/attachment]
[attachment=0]spectrum2.jpg[/attachment]

The color filter could be a very good base for the new Black & White procedure. Let's continue this titorial with the following example.
1. Open the "[i]rice-field[/i]" photo (from the previous post).
2. Go to [b]Image -> Adjustments -> Black & White... [/b]or jast press [b]Alt+Shift+Ctrl+B[/b]. The [i]White & Black [/i]dialog window appears.
3. From the "[i]Preset[/i]" combo box chose the "[i]Red Filter[/i]".
4. Uncheck the "[i]Preserve Luminosity[/i]" check box.
5. Move the "[i]Greens[/i]" slider left to the position [i]"-15 %[/i]". The "[i]Preset[/i]" combo box should automatically swich to "[i]Custom[/i]".
6. Move the "[i]Blues[/i]" slider right to the position [i]"40 %[/i]".
7. Press "[i]OK[/i]" button.
[attachment=2]BandW.jpg[/attachment]
[attachment=1]rice-field _ filtered.jpg[/attachment]
Please compare to simple desaturated image:
[attachment=0]rice-field_desaturated.jpg[/attachment]

Let us consider the Black & White adjustment more closely. Contrary to what one might initially assume, traditional black and white photographers actually have to be quite attentive to the type and distribution of color in their subject. Color filters are often used in front of the lens to selectively block some colors while passing others (similar to how color filters are used for each pixel in a digital camera's bayer array). Filters are named after the hue of the color which they pass, not the color they block. These can block all but a primary color such as red, green or blue, or can partially block any weighted combination of the primary colors (such as orange or yellow). Careful selection of these filters allows the photographer to decide which colors will produce the brightest or darkest tones.
We open an image:

[attachment=2]rice-field.jpg[/attachment]
Go to [b]Image -> Adjustments -> Black & White[/b]... or jast press [b]Alt+Shift+Ctrl+B[/b] for the [i]White & Black [/i]dialog window. At the top, the dialog window has [i]Preset [/i] combo box, where you can choose different color filters:

[attachment=1]mosaic.jpg[/attachment]
So which color filter is best? This depends on the goal of the mage, but in general: one can increase contrast in a given region by choosing a filter color which is complimentary to that region's color. In other words, we want to choose a filter whose color is on the opposite side of the color wheel to the image's color:[attachment=0]color.jpg[/attachment]

Creating black and whites is a great way to simplify or shift focus in a photo. You have seen great black and white photos out there in Magazines and in galleries on the web, Black and whites are everywhere. We will learn just how to make great black and whites in a matter of minutes. In Graff px we have 3 different black & white conversion techniques. We will start with the basic Desaturation of an image, learn why that is not-such-a-good way to create black and whites, and experiment with several different methods before settling with the best black and white conversion. After the conversion we will look at hue/satuaration/lighning, tweaking the channels, and subtle image toning. All of that is in this tutorial, watch, learn, enjoy!
Let's take a color photo:

[attachment=2]reds.jpg[/attachment]

[b]1. Desaturation.[/b]
[b]Image -> Adjustments -> Desaturate[/b] or [b]Shift+Ctrl+U[/b]
By changing it into black-and-white with method we get this picture:

[attachment=1]reds_1.jpg[/attachment]

It is rather insipid and boring. Even worse, the stripe of red flowers has almost disappeared from the field. This is when color channel filtering comes into question.

[b]2. Hue/Saturation.[/b]
[b]Image -> Adjustments -> Hue/Saturation[/b] or [b]Ctrl+U[/b]
Here we achieve the same result by manually desatuarating the image. In the dialog box jast roll the "Saturation" slider to the position "-100"
In comparision to the previous methid, here we have a possibility to play with brightness slider and make the resulting photo darker or brighter. But now let's go back to the color original!

[b]3. Back & White.[/b]
[b]Image -> Adjustments -> Black & White...[/b] or [b]Alt+Shift+Ctrl+B[/b]
A dialog window appears. Since we have an RGB picture, you can set the ratio of the color channels with the Reds, Greens and Blues sliders. Keep in mind that too extreme values may result in a radical amount of color noise. In this tutorial, I have used the following values:

Reds: 90%
Greens: - 40%
Blues: 50%

And here's the final picture! It has become much more dramatic, expressive, and interesting:

[attachment=0]reds_2.jpg[/attachment]