Assign Profile or Convert to Profile?

[Chimper]

What is the difference between these two Photoshop commands? I am currently putting together a portfolio where I need to provide two files of each image. The two files are to be as follows:

Tiff File - Adobe RGB, 350 ppi, 8 bit, unsharpened.
Jpeg File - sRGB, 72 ppi, resized to 14 inches on the long axis, highest Jpeg quality, sharpened.

The Tiff file will be used for publication, the Jpeg file for judging and evaluation. The Jpeg is converted from the Tiff and should look exactly like the Tiff. Is the "Convert to Profile" the command I should use on the Tiff when I make the Jpeg? Both of these commands change the file: "Convert to Profile" a very slight amount and "Assign Profile" quite a bit.

[jeffkohn]

You should use "Convert to Profile". It will preserve the appearance of the image (as much as possible) by converting the color values to the equivalent values in the 2nd color space. Assign to Profile doesn't convert any values; the only time you would really want to use Assign To Profile would be if you have a file that wasn't tagged with a profile and you knew which profile it should be tagged with.

[Chimper]

Thanks Jeff!

[Duffy Pratt]

Jeff's explanation is correct, except for the statement about the only time that you would use "assign to profile." It's something of an exception, but I have a series of profiles that I've created that I use at the start of processing to change the look of a file without changing the numbers. For example, if a picture is too dark, instead of forcing curves and levels to do all the work, I can start by assigning an AdobeRGB profile with a lower gamma (say 1.4 or even 1.0). This allows me to get a better starting point in trying to make something useful out of a dark original.

The whole idea of using "false" profiles in editing is something Dan Margulis talks about at length in a couple of chapters of his Professional Photoshop 5th edition. It sounds tricky at first, but its easy to play with and get the hang of. Also, with this technique, you can basically do the equivilent of the multiple raw exposure, high dynamic range processing, even if you are just starting with a jpg. You use assign profiles with different gammas to get your different exposures, edit these to taste, convert them to a common profile and merge.

Duffy

[jeffkohn]

Certainly there are other uses for Assign, but they won't come up very often (if at all) in most users' workflows. My point is that Convert to Profile is almost always the right choice if you're not sure.

The approach you describe is the idea behind Joseph Holmes' set of "chroma variant" profiles, which allow you to change not only gamma but color intensity. He wants 100 bucks for the set though, so I've never bothered looking into it, since I don't really feel that it would give me anything I'm currently missing from my toolset.