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When I merge 5 photos which are 5 MB JPGs into a panorama by choosing "Merge to panorama in Photoshop" from Lightroom, and save the resulting image as a .TIF, the file size becomes 650MB! Then, when I flatten layers and save again, it reduces to 111 MB.

Why does this happen? Is Photoshop creating new information? How can I save it in a format that doesn't lose any pixels, but also doesn't increase in size? The max size should be no more than 5 * 5 = 25Mb, right? Because Photoshop cuts out the images in the right pattern and stitches them together?

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    How large is the file if you resave as jpg? – xiota Jun 11 at 21:41
  • 9.7 MB if I resave as jpg with the max quality i.e. 12. But isn't JPG lossy in terms of colors? – greenberet123 Jun 11 at 23:08
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    JPG is lossy, but your source images are JPG, so it doesn't make sense to compare the sizes of 5MB JPG sources with a final uncompressed TIF. You can also resave the original JPGs as TIF for comparison, but I figured it would be easier to just resave a single output file to JPG than multiple inputs as TIF. – xiota Jun 11 at 23:30
  • Ah, that makes sense. I saved the individual images as jpg and it took 36MB. – greenberet123 Jun 11 at 23:52
  • When you transform a 5MB JPEG to TIFF, what effect does that have on the size? You're not creating new information, but you are storing it in a different way. – Michael C Jun 12 at 7:05
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Why does this happen? Is photoshop creating new information? ... The max size should be no more than 5 * 5 = 25Mb, right?

Your source images are JPG, so it doesn't make sense to compare their sizes with a final uncompressed TIF. (Apples to Oranges.) You will get a more reasonable output size for comparison if you resave your TIF output as JPG. (Apples to Apples.)

Then, when I flatten layers and save again, it reduces to 111 MB [from 650 MB].

Each layer contains pixels that were not present in the original to expand the image to fit parts of the panorama that the other photos cover. These pixels require space to store. Each layer also contains alpha channels to define transparent areas. Since you are using an uncompressed format, this "extra" information takes up a lot of space. Flattening the image discards "duplicate" pixels that aren't visible, as well as the alpha channels.

How can I save it in a format that doesn't lose any pixels, but also doesn't increase in size?

You usually cannot get photos down to the size of JPGs without resorting to lossy compression. You can try PNG, which doesn't support layers and is slower, but usually smaller than TIF. You can also save TIF with lossless compression – the best at this time is Deflate (Zip).

I would use the native format of the software (PSD for Photoshop, XCF for GIMP) while actively working on the image. Then save the final work with JPG quality 99 (Photoshop 12).

My experience has been that the difference between quality 99-100 and lossless is negligible. Plus you have the original source images if it becomes necessary to reprocess. While you would likely want to avoid duplicating work in the future, advances in technology should make future efforts quicker with improved results.

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Layers always increase images size, as photoshop tells you when saving. Depending on the original pixel size, the size of the Tiff is the total nuber of pixel x3 x nº of layers. But with LZW compression you get a lossless, nice tiff

  • Makes sense. Although, the LZW file for me was larger than without LZW. – greenberet123 Jun 11 at 23:53

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