Working with Photographic Standards and Scales, Part 1

Achieving accurate color reproduction when processing digital images is a topic that excites many photographers. In some cases, visual control during post-processing is sufficient to achieve perfectly acceptable results, but there are situations where such an approach is inconceivable. This is where various color charts and gray scales come in handy. In this article, I will share my experience using a three-step scale Danes Picta GC-3. After the simple gray card, this is the simplest reference used in photographic practice. It consists of three surfaces – white (ρ= 0.90)*, gray (ρ=0.20), and black (ρ=0.05).

An excellent example of using a three-step scale in practice can be a task that is as trivial as it is responsible - the reproduction of a painting. Look at the photos below. I asked three of my colleagues to process the raw file according to their subjective judgment, resulting in three different images.

Let's assume that the original is properly lit and correctly exposed. Next comes the processing, a process that should be approached carefully, without subjectivity and the desire to "improve" the image created by the artist. We can minimize the influence of the subjective factor by including a reference surface in the captured image. In this particular case, the reproduction was made with a Danes Picta GC-3 three-step scale, which is placed directly below the original.

Open the resulting file in Adobe Photoshop and go to Image > Adjustments > Curves. At the bottom of the window, you will see three eyedroppers for adjusting black, gray, and white. With the black eyedropper, you should point to the area of the image that should be the darkest. In this particular case, this is the black field of the three-step scale. There is one specific point here - the field in question is made of flock, which is fuzzy, and various particles may get caught on it. For this reason, you should be careful not to hit any speck of dust with the eyedropper. Then, with the gray eyedropper, you should point to the corresponding gray field of the reference, and finally, you should do the same with the white eyedropper and the white field.

The whole manipulation is simple and quick, but it has its subtleties. With the default settings, the black eyedropper sets a value of 0 in all three channels of the image (in RGB). The white eyedropper also sets extreme values – 255 in all three channels. In this situation, an unpleasant "clipping" occurs at both ends of the histogram, which is not at all useful for prepress specialists who will subsequently work with this file. We can avoid this effect by setting different values for each eyedropper. To do this, you need to double-click on the eyedropper and set the new parameters in the dialog box that opens. For the three-step scale, the manufacturer Danes Picta recommends the following values in Lab:
 

different values for each eyedropper. To do this, you need to double-click on the eyedropper and set the new parameters in the dialog box that opens. For the three-step scale, the manufacturer Danes Picta recommends the following values in Lab:

In some specific cases, you may need to set values different from those listed above. When photographing metallic surfaces with massive glares, you can increase the density of the white eyedropper to L = 94 or even less. For scenes with deep shadows, you can decrease the density of the black eyedropper to L = 8 or more.

Using the reference surfaces of the Danes Picta GC-3 three-step scale, we can impartially determine the density and color balance of the captured image. In addition to Adobe Photoshop, we can also use the scale with our preferred RAW converter. Most converters lack the ability to work with eyedroppers or adjust their parameters, but this is not a significant problem. Practically every RAW file processing software displays the RGB values for each pixel of the image. In this way, we can manually correct the image by tracking the corresponding values for each field of the three-step scale.

* ρ - a coefficient that characterizes the ability of a given surface to reflect part of the light that falls on it. For example, ρ=0.20 means that the surface reflects 20% of the light. 

 ©2008 Boris Bishkov, Dynaphos Ltd.