RGB vs. CMYK file sizes

[YrbkMgr]

Well, color gamut is usually smaller for CMYK so that may help explain it. But why is it you would have thought it was the other way 'round?

[E. Segen]

Well I have always found that when I change from RGB to CMYK the file size always goes up… at least the numbers at the bottom of the window go up (To be honest, I don’t pay much attention to the file size after saving). I don’t know but have always assumed that that the increase in file size was because of the information in the extra color channel.

I have absolutely no fact to back any of this up brilliant theory up with but I have never noticed the file size go down when I convert to CMYK.

[David S Sullivan]

I expected that because CMYK has four channels and RGB three, there would be more data in the CMYK file. Gamut makes a lot of sense, but is it that significant?

The RGB file (.psd) is 1.39 MB, the CMYK version is 306 KB. The RGB version is over 1 MB larger.

[E. Segen]

I would look at how the original was saved. I took a file that had been saved as a CMYK EPS image with jpeg compression maximum quality (3.12 MB) and resaved under a new name but as a TIF without any compression with a resulting file size of 11.2 MB, then I converted it to RGB and saved under a new name and that file size was 8.42 MB…

Was the original file saved in the exact same manner without compression on the exact same machine using the version of Photoshop that you used to resave, complete with or without a thumbnail version in the same format? I’m thinking that something has got to be different.

[David S Sullivan]

The two versions were saved from the same original. I created the file first in CMYK, and then changed mode to RGB and saved it with a different name.

I opened the CMYK file and Saved it as a TIFF with LZW compression and no layers. It came out at 298K.

From the same file, I changed mode to RGB and saved it as a TIFF with LZW compression and no layers. It came out at 696K.

This was on my PC. I tried it from scratch on my Mac and got similar results. Starting with one file, I saved it in CMYK and RGB. The CMYK file (8.3 MB) was smaller than the RGB file (10.3 MB).

Could this have anything to do with a CLUT being saved in the RGB file but not in the CMYK?

[David S Sullivan]

Sure enough. The uncompressed CMYK TIFF is 8.89 MB and the uncompressed RGB TIFF is 6.67 MB.

Thanks!

[YrbkMgr]

Harv... brilliant, as usual.