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Color to Black and White
- Introduction -

There must be as many ways of converting a RGB image to black and white as there are photographers.  There are entire web sites devoted to the many ways of doing this.  Many of these sources show us a technique to convert a RGB image to black and white and then state 'move the sliders around until you see something you like'.  While is it correct that the final image is more important than the original image, it always seemed to me these techniques regulated digital black and white photography to the electronic equivalent of an Easter egg hunt - we keep looking until we find something interesting.  For those who have never made their own black and white prints before, these techniques can seem powerful.  However, if you are the type of black and white photographer who visualizes their final image and makes their own black and white prints, these customary techniques may not offer the methodical approach you are used to.

I find that I get to where I want to be much quicker and easier when I know where I am starting from.  So when I started researching how I wanted to convert a color RGB image to a black and white RGB image, I looked for several things.  One, a consistent method that accurately converted the tones in the color image to black and white tones as defined by the photographer.  Two, after the black and white conversion, I still had full control of tonality by color.  Three, the method was predictable and intuitive.  Four, the method was non-destructive.  Five, and most important, the method fully supported a visualization approach to creating a black and white image.

In my research, I never ran across a method that met all the criteria.  So I devised my own.  I call it the Tone Management System, which is discussed on the Tone Management System page.
 

Accurate Tone

Choosing a method that creates accurate tones is more difficult than one would seem.  What is the definition of accurate tone in digital photography?  Is it %K, or percent black, as shown in the Adobe Photoshop Info panel?  Is it the value shown by the Curves control?  Is it the grayscale tones if the image were converted to a true grayscale image?  Is it the lightness channel of an image in the Lab color space?  Each of these will give different results when applied to the same image.


On this web site, I describe how to use the Tone Management System to create accurate tone for each of the four definitions above.  So, what we need to do is decide what our definition of accurate tone is.  Is it %K?  Is it the Curves value?  Is it Grayscale tones?  Or, is it the tones as seen in the lightness channel of the Lab color space?  Unfortunately, I cannot give a step by step guide to help someone make that decision.  My personnel baseline for determining accurate tone conversion is the Photoshop Curves control.  If the Curves control shows that for a certain area of a color RGB image the value is 75 and the Curves control shows that the value for the same area in the black and white version of the RGB image is also 75, then I consider the tone to be accurately converted.  If you cannot decide which definition of accurate tone meets your needs, I recommend you study all the Tone Management System methods, view the examples, and then make a decision that seems right for you.

Note

Accurate tone is not about replicating exactly the tones in the scene being photographed, unless that is what the photographer wants.  Accurate tone is the base digital luminance as chosen by the photographer.

 

Managing Black and White Luminance by Color

Color Wheel
Figure 1. Color Wheel

There are two basic ways of targeting color in Photoshop: color ranges and color channels.  A color range is a series of one or more colors that are contiguously located on the color wheel.  See Figure 1.  An example of a narrow color range is the reds.  An example of a broader color range is the cyans-blues.

The Photoshop Select > Color Range command and the Hue/Saturation adjustment allow us to select by color range.

A color channel is the color and tonal information for one of the colors in a color model.  For example, the RGB color model has three color channels; one for red, one for green and one for blue.

 

The Photoshop Levels and Curves adjustments allow us to select a specific color channel and make adjustments just for that color channel. 


As we will see later, we can use the combination of the Select > Color Range command and Levels/Curves to allow us to make adjustments to color ranges using Levels and Curves.
 

Being able to select by color range or color channel gives us another option to target areas of our image to modify their tone.  It is this level of control that I wish to maintain in my black and white photography.
 

Predictability and Control

Without going into Statistical Process Control, Six Sigma or other process improvement methodologies, there is one thing they all have in common.  If we cannot reasonably predict what is happening in the method or process we are using, then we really do not have control.  So, regardless of the black and white conversion method, if someone just moves sliders around until they see something they like without knowing ahead of time what is going to happen when they move the slider, then their black and white method is more akin to opening presents at Christmas; they don't know what they are going to get, they just hope it is going to be something they like.  Predictability and control go hand in hand.  I want a technique that I can use in a predictive and controllable manner when working on an image.
 

Intuitive

Simply put, a method that is intuitive is a method that makes sense to the user.  It is true that intuitiveness varies by person.  And it should also vary for a person as they gain experience.  For example, using the Curves control to a new Photoshop user is probably not very intuitive.  But with experience, one would hope that the Curves control will become intuitive.

I will also admit that managing tone by color in a black and white image may not seem intuitive.  However, experienced black and white photographers have used color filters to darken a blue sky or green foliage for decades.
 

Non-destructive

Non-destructive methods are any computer based image editing technique that changes an image's appearance without permanently altering the image's pixels.  They allow us to 'undo' a change even years after the change has been made.

 

Visualization

Simply stated, visualization is the art of seeing the final image in one's mind and then using traditional and digital photographic techniques to create it.  I do not want to settle for what Photoshop automatically gives me.  I want to use Photoshop, or any other image editing software, to create the image of my choosing.