Color Management: Converting versus Assigning
Every color-managed image has a profile associated with it that defines the color space in which the document lives. What if you want to change an image to a different color space? We already know that this is precisely what happens when you use your printer profile to print, but there are other times you might need to do this. And when you do, you will undoubtedly encounter the dilemma of whether you should use Convert or Assign. Both of them live on the Image >> Mode menu in Photoshop. They sound a lot alike, but what they do is quite different.
Converting to a profile changes the raw data in your image (based on your Rendering Intent) so that it looks as close as possible in the new color spaces to how it did in the old color space. Think of this as being just like converting money from one currency to another. If you have $100 US and you convert it to Canadian, you would now have $131 (as of this writing). The data changes, but it's still worth the same amount.
Assigning a profile merely changes how the existing data is interpreted by telling your color management system that it is now in a new color space. It does not change the data and therefore does not use Rendering Intent at all. It is guaranteed to change the way your image looks. Assigning is similar to taking your original $100, crossing out the words "United States" and writing in "Canada." The data does not change (you still have $100), but it's no longer worth the same amount. If you could figure out how to spend it now that is. But hey, it's an analogy.
Why would you choose one versus the other?
Converting is appropriate when the image looks good already and you want to keep it that way, but need to change color spaces. For instance, if your working space is Adobe RGB and you want to create a jpeg to post on the web, you need to convert your image to sRGB, the standard color space for web images.
Assigning is used less frequently, but comes in handy when you open an image that lacks a profile or is incorrectly tagged. Here's one real life scenario. I have a profile for my Nikon 4000 ED made with Monaco/X-Rite EZ Color. When I open a scanned image from NikonScan into Photoshop, it doesn't not have an associated profile (Nikon Color Management in the scanner software is rather limited). I then assign this profile to the data to make it look right. To get the image into my regular working space, I then convert to Adobe RGB.
Next week I'll try to address some of the more common problems you may encounter as you learn about color management.
Update 5/01/2005 - With the advent of Photoshop CS2, Adobe has moved both Assign and Convert over to the Edit menu rather than the Image menu. So now we have Edit >> Assign Profile, and Edit >> Convert to Profile. Saves a mouse click I guess, once you get used to it.