Assign Profile… Convert to Profile…
These are two of the most powerful commands in Photoshop—and among the least understood. They sound interchangeable. They sound compatible. Yet, even though they’re related, they are worlds apart in function. My students and clients often get into trouble with these evil twins. Let’s see if we can pull them apart and demystify their purpose.
The Role of Color Management
Let’s first deal with the concept of profiles and their role in color management. Color management, for all its grand complexity, does really only two things. Firstly, it standardizes the RGB and CMYK values of various devices and processes and turns them into something that relates to the actual functions of the human visual system. That is, it takes ambiguous numbers and makes them unambiguous. This gives quantifiable meaning within the context of human vision. Let’s take a computer’s display screen as an example. One particular display’s RGB values might not even be close to that of another. And no display’s RGB values will match that of a camera or a scanner or any other device. It’s a big mess, a mess we have to get under control if we have any hope at all of producing consistent color. Therefore, taking RGB or CMYK values and standardizing them is the first goal of color management. All RGB and CMYK values are converted to XYZ or LAB values which, unlike RGB and CMYK, are unambiguous and based upon how we actually see and perceive color.
For the sake of this discussion we’re going to avoid getting into a deep conversation of XYZ or LAB colorspaces. Suffice it to say that they are derived from detailed study of the human eye and human vision. Since human vision and human color perception are what matters most in our imaging processes, XYZ and LAB are the final arbiters in the process. Their numbers are firmly rooted in reality and therefore any valid color system must relate to them and standardize to them.
It’s all rather basic at its essence. RGB and CMYK give us colors that are dependent upon our devices and they vary. XYZ and LAB are models based on human vision and are device independent. The job of the profile is to correlate the numbers that our devices give us (RGB and CMYK) to a standard that won’t ever vary.
Once we standardize our devices we still have one more basic problem. The primary colors for these devices will still vary. Yes, they’ll all be correlated to a common standard but the red of an Apple display will still be different than that of a Nikon camera. However, we really don’t care as long as we have a consistent way of preserving the appearance of our images across all devices. In order to do that we must change the RGB or CMYK numbers in order to preserve the appearance throughout the process. That is what convert to profile is all about.
Convert or Assign?
If we care about preserving the appearance of an image, we use the Convert to Profile… command in Photoshop. We use this when we don’t know what the profile is or we wish to use a different profile for our own reasons (another topic for another day). If we care more about preserving the RGB or CMYK numbers in our file, we use Assign Profile… instead. But, here’s the catch: the RGB numbers will indeed be preserved but they will be correlated to different LAB values which will make them look different. So, we generally assign profiles when a file comes to us untagged, meaning that there is no embedded profile assigned to the image. It’s like a refuge without a country.
Problems occur when people start assigning profiles when they actually wish to convert to profiles. Converting will preserve appearance and change the RGB or CMYK numbers. Assigning profiles will preserve the numbers in RGB colorspace (CMYK is more complicated than that but that is also another subject for another post) but will change the appearance. Some people see the changes that profile assigning will cause and get confused and misinformed about the actual profiles themselves.
If this all seems as clear as mud, have faith. You are not alone. But, you must persevere. There was a time when color management was relegated to dedicated professionals who worked with color their entire life. Now all of us need to understand this stuff. Little by little it all starts to make sense. We’ll continue to explore the magic and mystery of color. And if you have any questions, please contact us via our Imaging Help Desk. We’re here to help