Introduction to Color Calibration for Digital Photography

ColorChecker Classic Macbeth Chart

The following is a brief tutorial to describe the basics of color calibration for novices. This is not meant to be a comprehensive overview of color calibration but briefly introduces beginners to different concepts and their application. The resulting output should not be considered “color accurate” but a decent approximation and easily obtained with readily available and affordable software.

Color calibration is a process that adjusts the color response of an input or output device to a known state. It’s crucial in digital photography to ensure the colors in the final image align with the original scene. The Color Checker Classic is often used for this purpose. It contains 24 patches of known colors that can be used to adjust the color response of the camera. While commonly used, this chart does have limitations…

It is important to note that for this tutorial and technique to work properly that the light sources must be known illuminants. What is an illuminant? An illuminant is a mathematical representation of a theoretical light source. In the case of Adobe DNG Profile Editor, it supports two illuminants. The first is 2850K which is typically referred to as Warm White which is a bit warmer than the standard incandescent bulb. This color temperature is often used in living rooms, dining rooms, bedrooms, and other areas where a softer light is preferable. The other is 6500K or Daylight where this color temperature gives off a bright amount of blue-white light, similar to natural daylight. The Adobe DNG Profile Editor can create a dual illuminant color table, taking both illuminants into consideration such as a room with large windows and mixed lighting conditions. If you have a light source which varies from these known illuminants, other techniques and software will be required as the mathematical transform can produce undesirable results. Stay tuned for a more advanced tutorial coming soon.

Now, let’s understand some key definitions in layman’s terms:

Color Space: A color space is a method for organizing colors in a way that allows for their representation. It functions similarly to a map, guiding us to each specific color. For instance, the RGB color space represents colors through a mix of red, green, and blue values. Various color spaces can depict different spectrums of colors. They establish a standardized method for defining colors numerically within a certain scope. Adobe RGB and sRGB, for example, are two distinct absolute color spaces derived from the RGB model, with some color spaces offering a broader range of colors.

RGB Color Model with Adobe RGB and sRGB gamut ranges

Color Gamut: This refers to the complete range or subset of colors that can be accurately represented or produced by a device or a specific color space. In other words, it’s the total palette of colors that a device can reproduce, or a color space can represent.

So, the correlation between color space and color gamut is that the color space defines the framework or system for representing colors, and the color gamut is the range of colors within that system that can be displayed or reproduced. For instance, an sRGB color space has a certain color gamut, which is the range of colors that can be represented within the RGB color space. sRGB color space is the most common and is used in most display devices.

It’s important to note that different devices (like monitors, printers, cameras) may have different color gamuts, meaning they can represent or display different ranges of colors. In visual effects, film and television, ACES is a common color framework with numerous color spaces/gamut for specific use cases. For example, ACEScg is used for floating point 3d rendered images, ACES AP0 encompasses the entire visual spectrum and used primarily for archival and interchange usage while ACES AP1 is intended more for final production with a representation proportional to visual perception. For more information on ACES, visit this link: ACES “working” Spaces — ACESCentral

Gamma: Gamma, also known as the response curve, is a concept in imaging systems that defines the appearance of mid-tones between pure black and pure white. Unlike linear brightness or contrast adjustments, gamma is an exponential function and a form of contrast adjustment. Practically, gamma adjustment alters the luminance of mid-tones without significantly changing the shadows and highlights. This is vital as our eyes perceive light differently across various brightness levels.

For instance, if an image appears too dark on a specific device, adjusting the gamma value can make it look brighter without compromising the highlight details. Thus, gamma or response curves offer a sophisticated method to modify an image’s brightness and contrast, ensuring it appears correct and natural to the human eye on any display device.

When generating a DNG profile, it is common to set the gamma response curve to linear. This means that the luminance values from black to white are evenly distributed across a straight line from 0 to 1. This approach provides the most flexibility for adjusting the gamma curve later in post-processing tools such as Adobe Lightroom. However, please note that an image with a linear response curve may appear flat or washed out when viewed on a standard sRGB monitor. This is because sRGB monitors apply a gamma correction, and they expect images to have a certain amount of contrast that a linear image does not have

Adobe DNG Profile Editor with camera base tone curve and base profile curve in red

Adobe DNG Profile Editor with built in camera base tone curve and linear curve in red

White Balance: White balance is a camera setting that adjusts the colors in a photo to accurately represent how white should appear under different lighting conditions. It ensures a neutral color tone and helps to create true-to-life colors. It does this by compensating for the light source’s color temperature. Color temperature is measured in Kelvin. The following image is an example of color temperature ranges for different lighting conditions.

Color Temperature Chart by Lighting Condition Measured in Kelvin

Now that you understand some basic concepts, let’s explore how we can use a Macbeth chart and the steps required to calibrate the color for your photography.

Here are the basic steps of color calibration using a Macbeth chart and generating a DNG camera profile:

1. Purchase a Macbeth Chart. The Calibrite Color Checker Classic is the most commonly used chart in VFX production.
2. Place the Macbeth chart in a properly illuminated location. Often two color charts are shot, one in direct sunlight and a second in shadow. You color calibrate for daylight, then find the average of the white balances between the two images when processing the images in Lightroom.
3. Adjust camera setting for proper exposure and white balance, being sure to select the native RAW format for your camera. Be sure to set your camera settings to manual, with a low ISO (to reduce noise) and use the same camera settings for subsequent shots. If you need to change camera settings, get another color chart pair shot.
4. Select the color space in the camera to the widest available color gamut such as AdobeRGB instead of sRGB.
5. Shoot the color chart perpendicular to camera view direction straight down if possible. Placing the chart on the ground will give you a spherical lighting from the sun and sky. When placing the chart, avoid being too close to other surfaces. A red house will bounce light, even in shadow and that will affect the final calibration. For photogrammetry, never hold the chart or place on vertical surfaces unless you are specifically trying to get color calibration from a specific view angle.
6. Avoid oblique, angled view shots of the chart as specular highlights may introduce variance in the luminosity of many of the color chips. This will affect your final calibration so it is recommended to avoid this when possible.
7. Download and install Adobe DNG Converter and DNG Profile Editor from this web page URL: Digital Negative (DNG), Adobe DNG Converter | Adobe Photoshop
8. If the RAW image is not in the DNG format, convert the RAW image to DNG using the Adobe DNG Converter.
9. Open DNG image in Adobe DNG Profile Editor.
10. Assign camera gamma or response curve to linear.
11. Arrange color chip anchors to corresponding chart chips.
12. Choose the appropriate illuminant based on lighting conditions (2850k for Warm Light, 6500K for Daylight or both color tables if you have a mixed lighting scenario with both light sources such as a room with large windows in daylight.)
13. Hit Create Color Tables.
14. Generate and save the DNG profile.
15. Load & apply DNG profile in Adobe Lightroom.
16. Set white balance and adjust image as desired.

Here is a short video demonstrating Steps 7 through 16. The camera used is the iPhone 14 Pro Max, shot in DNG format with the Apple ProRes color space:

This concludes the tutorial on an Introduction to Color Calibration for Digital Photography. Stay tuned for more advanced lessons coming soon.