Color Matching with Adobe Camera Raw 3

April 21, 2006

Update (July 2008)

It's also possible to use the DNG Profile Editor to get Camera Raw / Lightroom to match the color appearance of other (non-Canon, non-Nikon) raw converters, or even do funky things like make a Canon camera look like a Nikon camera! See Tutorials 2 and 3 on the DNG Profile Editor online documentation page for more info.

For the time being I've decided to leave the original text of this article online.

Overview

When I come across an image that gives me color headaches in ACR, I often try using other converters. For example, I often find that Capture One gives me a very pleasing color treatment with very few adjustments, especially with skin tones. Of course, having to switch back and forth among multiple tools is annoying and disrupts the workflow. So I began wondering: wouldn't it be nice if I could imitate Capture One's color rendering using ACR? Or, more generally, is it possible to use ACR to reproduce the color and tone curves of other RAW converters?

Autumn Color, Vermont, 2004
(developed in ACR using a "Capture One style")

The answer is yes -- or, at least, you can get pretty darn close! The process is reasonably straightforward and involves the following three steps: first match the white balance, then match the tone (using curve points in ACR's Curve tab), and finally match the color (using ACR's Calibrate tab). It takes some time to complete these steps, but the nice thing is that once you're done, you can save the resulting calibration (and optionally the tone curve, too) so that it can be easily applied to other images with the click of a button.

The rest of this page is devoted to describing the color and tone matching procedure in more detail, the idea being that if you've read this far, you're probably wondering how you can do this yourself. I often hear comments of this nature from other photographers: "I like ACR, but I prefer the color rendering of DPP" (or some other RAW converter). Well, hopefully this article will help you coax more pleasing colors out of ACR.

What follows is a practical guide with little explanation of the theory and the principles. If you wish to learn more about color science and its role in photography, I suggest visiting Bruce Fraser's online articles and Bruce Lindbloom's web site.

To carry out this procedure on your own, you will need three items:

• Adobe Camera Raw (version 3 or later, comes with Photoshop CS 2),
• a camera supported by ACR, and
• a GretagMacbeth ColorChecker Chart

Profiling: step by step

1. Photograph the ColorChecker under your desired illumination conditions. Some suggestions:
   • Be sure to expose in RAW mode.
   • Expose at a low ISO (e.g. 200).
   • Avoid nearby colored objects (e.g. grass, painted objects, etc.), because these may leave a color cast on the chart.
   • Avoid including shiny objects (or anything else brighter than the white square) in the frame.
   • Keep the illumination on the ColorChecker as even as possible.
   • Expose carefully to avoid clipping the white patch and the dark patch. Use your camera's histogram to assist you.
   • Stop down the lens (e.g. f/11 or smaller aperture) to minimize vignetting. It's fine to avoid completely filling the frame with the chart.

2. Copy the RAW image of your photographed ColorChecker to your computer and open it using the target converter. In the example image below, I've opened the RAW image in Capture One:
   
3. Set the white balance in your target converter using the gray square next to the white square (R4C2). In Capture One, this step is performed by selecting the White Balance tab, then clicking on the gray square:
   
4. Process/develop the RAW image using your target converter. In particular,
   • Set the destination color space to ProPhoto RGB. This will be the working RGB space of the converted image.
   • Save the output image in a lossless format, such as TIFF.
   • It is not necessary to use 16 bits per color component; 8 bits is fine.

   From now on, we'll call this converted image the target image.

5. Open the target image in Photoshop and use the color sampler tool to find the RGB values of each of the gray patches in the bottom row of the ColorChecker chart. Jot these numbers down.
   

   Note that since these squares are supposed to be neutral, the color components for a given square should be pretty close to one another. You might also notice that the values vary depending on where you sample the square. Small changes here and there are usually due to noise. If these variations bother you, just select an area near the center of the square, run the Blur -> Average filter, and then obtain a sample from within the selection area.

   Here are the numbers I got in my case: 224, 191, 144, 92, 47, 21.

6. Now we'll begin working in ACR. The idea is to match the white balance (using one gray square), then the tone curve (using all six gray squares), and finally the colors (using the red, green, and blue color squares).

   Use ACR to open the RAW image of the photographed ColorChecker:

   
7. Use the crop and align tools in ACR to exclude everything else except the 24 squares of the ColorChecker (i.e. eliminate all background material). The purpose of this step is to make the histogram accurately reflect the squares' tonal values. You may want to zoom in after performing this step to make the ColorChecker image easier to see.
   
8. Perform the following setup in ACR:
   • Choose "ProPhoto RGB" as the color space in ACR's workflow options.
   • In the Adjust tab, set Exposure to 0, Shadows to 0, Brightness to 50, Contrast to 25, and Saturation to 0. (These are ACR's defaults, except for the shadows slider.)
   • In the Detail tab, set sharpness to 0, luminance smoothing to 0, color noise reduction to a moderate value (e.g. 20-25).
   • Set all values in the Lens tab to 0.
   • In the Curve tab, choose "Linear" from the Tone Curve menu.
   • Set all values in the Calibrate tab to 0.
   

9. Look at the histogram and make sure that nothing is clipped, i.e. no overexposure and no underexposure. You can check the highlight and shadow warning boxes to make sure that the white square is not overexposed and that the dark square is not underexposed. If overexposure or underexposure has occurred, try another exposure of the ColorChecker with your camera.
   
10. Press S to get the sampler tool. Add "sampler points" to the middle of each square in the bottow row (the gray patches). Also add sampler points to the leftmost three patches in the third row (blue, green, red patches). Sampler points are a great new feature in ACR 3.
    
11. Set the white balance (use shift-click with the sampler tool, or press "I" to get the white balance tool) using the patch in row 4, column 2 (i.e. the gray patch to the right of the white square).
    

    Move the sampler tool over the surface of that gray patch (R4C2), keep an eye on the RGB readout values, and make sure that the RGB values are fairly neutral. They should not be more than one level apart (e.g. 189 190 190 is ok).

12. Go to the Curve tab. Look at the sampler points that you added for the six gray patches. Ctrl-click on each one to add a corresponding point on the tone curve. There should now be six interior points on the curve, corresponding to the six gray patches.
    
13. We are now going to match the tonality of the target image. While keeping an eye on the six gray patch sampler values near the top of the ACR window, adjust the six points on the tone curve so that the sampler values match the gray patches' values in the target image (see Step 5, above).

    The easiest way to do this is to use ACR's keyboard shortucts. Use ctrl-tab (or shift-ctrl-tab) to select the next (or previous) control point, and use the up/down arrow keys to adjust the tone values. Hold down the shift-key to make increments of 10 instead of 1.

    If the patches' RGB values aren't perfectly neutral, that's ok; just match the green values.

    
14. The last step is to perform color matching. Here is the iterative approach I used.
1. Start with the green patch (row 3, column 2) and try to get the values to be proportional to the target image's values for the green patch. In my example, the green patch in the target image has RGB values (80, 112, 69). Since the green sampler in ACR shows that the green value is 117, I aim for (84, 117, 72).

   Use the green saturation slider to adjust red and blue values relative to the green value. Use the green hue slider to adjust red and blue values relative to each other. Tweaking the hue slider will sometimes cause the green saturation value to change (e.g. from 118 to 117). That's ok. Just keep tweaking the hue and saturation sliders until the green sampler shows values that are proportional to the target image's green patch values.

   For my example, here is what I got:

   

   Notice how my values for the green sampler (sampler 8) have the same ratios of R to G to B (84 to 117 to 72) as the green patch in my target image (80 to 112 to 69). For your own case, the necessary green hue and saturation slider values may be entirely different from mine, so don't simply enter the slider values from my example image above!

2. Next, switch to the blue patch (row 3, column 1) and its sampler. Repeat the process with the blue saturation and hue sliders. The goal is to get the values of the blue patch in ACR to be proportional to the blue patch values of the target image.

   Here is what I got:

   

   After this step, you'll probably find that the sampler values for the green patch have changed and may not be balanced properly. That's ok. Don't worry about this for now; we'll come back to it later.

3. Repeat the process for the red patch (row 3, column 3) and the red saturation and hue sliders. Here is what I got:
   

4. This completes one round of color slider tweaking in the Calibrate tab. As I mentioned above, the red sampler might now show a balanced set of values, but the green and blue sampler values are probably off. In my example above, you can see that this is indeed the case. They shouldn't be off by too much, though. By repeating the above three steps, we'll quickly converge on a balanced set of red, green, and blue patches.

   Fine-tune the sliders by repeating the above steps, starting with green, then blue, then red. In most cases you shouldn't need more than three or four iterations. Once you become comfortable with how the hue and saturation sliders work in the Calibrate tab, this iterative process should only take a few minutes.

   After 3 more iterations, I converged to this:

   

   While the values of the red, green, and blue patches don't match exactly with the ones produced by the target converter, the have the same color component balance (i.e., the ratio of red to green to blue within each patch is about the same).

15. Wow, that was a lot of work! Better not let it go to waste, then. Save your calibration settings by going to the right-triangle pop-up menu and choosing "Save settings subset ...".
    

    In the dialog box that comes up, choose "Calibration" from the Subset menu and click the "Save..." button. If you like, you can also save the tone curve used by checking the "Tone Curve" box, too.

    Give the target .xmp file a descriptive name, such as "Canon_5D_daylight_CaptureOne.xmp".

    Now whenever you want to apply the color rendering of your target converter to an image, all you have to do is choose your saved file from the Image Settings menu in ACR! If you also saved the tone curve in the .xmp file, you'll match the tonality, too.

Example

Here's a photograph I took in Vermont in October 2004. The tree's leaves had a deep red color, and I recall struggling with ACR to obtain the same richness that I saw when capturing the image. The default rendering from ACR does a pretty good job with the image's tones, but the reds look rather lifeless.

Default output from ACR (no calibration). Reds look rather pale.

For the image below, I set the calibration sliders using the calibration technique described in this article. The reds certainly come to life, though they are slightly too pink for my taste, especially in the leaves in the upper-left part of the frame.

Output from ACR after applying the color calibration method described in this article. Reds are more vibrant but have a slight pink/magenta cast. This might be fixable with a gentle White Balance tint tweak, but not without affecting the green foliage, too ...

For the third image, I used the tone curve and calibration settings obtained using the color-matching procedure described above. In other words, I used ACR to produce an image in the "style" of Capture One.

Output from ACR using the color matching approach described above, i.e. in the style of Capture One. Of the three images here, this is the one I prefer.

Which is the best? There's certainly no right or wrong answer here. These images are just three different interpretations of the same RAW image data. I actually expect some readers may prefer the second (middle) image. Personally, I like the third image best; its colors and tones are the most pleasing to me and most closely represent the scene as I remember it. Perhaps most importantly, they remind me of why I wanted to photograph that tree in the first place!

Now, I'm not saying that going through this tedious color-matching procedure is the only way to get pleasing colors in ACR. Far from it. In this case, I was having some difficulty getting the color balance I wanted in the image, and I found it easier to get there by imitating the colors produced by another RAW converter. This technique is not intended to replace standard curve and slider adjustments in ACR. It's simply another tool to add to one's digital darkroom toolbox.

ColorChecker Comparison

Below you can see the photographed ColorChecker chart developed by the target converter (Capture One). Move your cursor over the chart to show the one rendered by ACR after color matching and tone matching. (I have converted both of these images to sRGB for web display.) Not identical, but pretty close.

Move your cursor over the image to switch to ACR's output.

Closing Thoughts

The color matching approach should generalize fairly well across all RAW converters, not just Capture One. In my own case, I've tried it on Capture One, Raw Shooter Premium, and Canon's Digital Photo Professional (DPP) software, and it has worked well in all cases. Again, it's best not to think of the results as "right" or "wrong" -- it's a matter of taste, after all. The calibration settings I obtained by matching Capture One, for instance, work better for some images than for others.

I must say, though, it's really nice having all of these different color and tone variations available in a single interface. I just cycle through the Image Settings menu until I find one I like and use it as a starting point for finer adjustments. It's like having a little "appearance and style" library right at my fingertips.
 

Acknowledgments

Thanks to Jeff Schewe for helping me get started with the color matching procedure.