Today’s Question: What is the difference/advantage of changing a file to sRGB via “assign profile” and “convert to profile” [in Photoshop]?
Tim’s Quick Answer: If you want the colors in the photo to remain unchanged (to the extent possible), you should use the “Convert to Profile” command. If you want to change the appearance of colors in the photo based on a profile, you would use the “Assign Profile” command. In general I would say that photographers today are therefore more likely to want the “Convert to Profile” command, and not the “Assign Profile” command.
More Detail: There are two basic reasons you might want to change the color space associated with a photo. The first (and most common for photographers these days) is to change a photo to a different color space based on a specific output scenario. For example, when preparing a photo to present online, it can be a good idea to convert to the sRGB color space to help ensure more accurate color for the photo being shared. In this type of scenario you want to maintain the same appearance of colors in the photo.
The other scenario is one where you actually want to change the appearance of colors based on a profile. For example, if you scan a slide or negative, you could use an ICC profile for the scanner to apply an automatic correction to the scanned image. The scanner profile would then be used as the basis of a change in color appearance for the photo to make the resulting colors more accurate.
These days I find that many photographers have a need to convert a photo to a different profile for reasons related to how that photo is being shared. In this type of situation you want to maintain the color appearance of the photo, so the “Convert to Profile” command would be appropriate.
Only when you actually want to change the appearance of a photo based on an ICC profile should you use the “Assign Profile” command. These days I would say that most photographers are therefore probably not using the “Assign Profile” command much, if at all.