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When working on several Photoshop PSD documents that have the same document dimensions (size and resolution) in the same RGB colour space, why do they end up with different file sizes when flattened to a TIFF format with no compression, using the same saving features?

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  • How big is the difference? A small difference might be explainable by metadata. (Also be aware that a small difference in actual file size can cause a bigger difference in size-on-disk, if it makes the file require an additional block.)
    – Wrzlprmft
    Feb 16, 2015 at 22:14

3 Answers 3

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Different areas within a document need different bytes. File dimensions are only one aspect of a file's size.

A pixel that is 0/0/255 needs a different set of data than a pixel which is 199/238/175.

Unless every pixel in your documents match, they will always be different in terms of file size.

3 tiffs....

all 500x500px flood filled with a color and saved with the exact same steps. The only variation is the actual pixel data. In fact, the only variation is the color of the document. No embedded color profiles, layers, or transparency.

enter image description here

The sizes will vary based on the pixel data.....

enter image description here

Right-click/Control-click and open image in a new tab/window to see it easier

This slight variation, could be due to the embedded preview image for the file. If the preview is using compression, then the different color fields could be compressing differently. For solid, irrefutable, answers as to why this happens, I think you need to ask Adobe as it's a tech support issue, not a design issue.

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  • Your explanation assumes that TIFF stores colour information as a plain text. But this would be unnecessarily inefficient and is nothing I could confirm at a quick glance or by looking at some actual TIFFs. If TIFF however stores these numbers as binary data (which I strongly assume, because that’s how you store data efficiently), 0/0/255 and 199/238/175 would indeed take the same amount of space, since they both correspond to 3 bytes. There is a reason that each colour axis can have 256 = 2⁸ possible values, you know.
    – Wrzlprmft
    Feb 16, 2015 at 22:01
  • TIFFs store data in a variety of ways and here
    – Scott
    Feb 16, 2015 at 22:06
  • Yes, but this question is about TIFFs saved with the same settings and even then, this is not what you explained.
    – Wrzlprmft
    Feb 16, 2015 at 22:11
  • No 2 files are ever going to be the exact same size unless the files are identical in every conceivable way. All 3000x1500px, RGB, tiff files containing identical metadata and color profiles are never going to be the same size (kb) if there is any variation in the pixel data.
    – Scott
    Feb 17, 2015 at 1:23
  • That’s just wrong (even if we exclude sheer luck). Not only is there no reason why this should be so (as explained above); I also tested this just to be sure: I created some doodle with GIMP, exported it as a TIFF with no compression, doodled some more and exported to another TIFF with exactly the same settings and voilà: Exactly the same file size.
    – Wrzlprmft
    Feb 17, 2015 at 8:25
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If you have maximize compatibility set to on then PhotoShop creates a hidden flattened layer.

If you don't need backward compatibility you can turn it off. Choose Edit > Preferences > File Handling.

Change the 'Maximize PSD and PSB File Compatibility' to 'Never' and Photoshop will stop asking you every time you save a new file.

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Without knowing the magnitude of the differences you are seeing, it is hard to say. The most likely culprit (aside from metadata differences mentioned by Wrzlprmft in their comment) is a thumbnail/preview which is probably compressed. Depending on the image, these can vary in size. Inclusion of thumbnails is a set-able application preference and also a dialog option for TIFF.

It is plausible that there is some sort of packing algorithm involved when "no compression" is chosen, but I haven't seen any explicit reference to this aside from "pack data into bytes as tightly as possible" in the TIFF docs.

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