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Using Lightroom + Photoshop for focus stacking

I have a 42MP Sony A7R3 camera, and I used compressed RAW, which results in ARW files of around 42MB.

Say I have 2 of these files, and I open them as layers in Photoshop, focus stack them, and then flatten them to remove all layers. The resulting image contains the same number of pixels as each individual image that I started off with.

Now, I am trying to save this, to continue editing in Lightroom, but no matter what format I use i.e. PNG or TIFF, the resulting file becomes over 200MB and I don't have that kind of space! How can I get the focus stacked image back to a reasonable file size of 42MB that I started out with?

Thanks!

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  • \$\begingroup\$ If you need a "resulting file", why you do not export it to a flattened file? \$\endgroup\$
    – Rafael
    May 17, 2023 at 16:34
  • \$\begingroup\$ just save it as a jpeg, 90% quality. you can compare it to a tiff side-by-side, but you'll be hard-pressed to sight any meaningful difference, especially if you're not re-compressing it multiple times. I once made a PS action to resave the same image 100 times at 100% quality and every pixel's RGB was the same at the end; literally lossless. The diffs were still tiny at 90%, but below that, there are (minor) artifacts and file size doesn't diminish as much as the first 10%, so stick with 90%... \$\endgroup\$
    – dandavis
    May 24, 2023 at 1:04

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42MP, 3 8-bit channels, is 126MB (I assume there is very little compression).

If you work from a raw file, PS may be trying to transfer as much data as possible using PNG with 16-bit color channels, so your file is potentially 252MB (TIFF could be even worse, storing floating point data on 4 bytes/channel).

In 2022, not having 400MB to spare on a system where you handle 42MB files is not a normal situation.

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  • \$\begingroup\$ I don't like to waste space in a meaningless way. My question is, when Sony can fit the same number of pixels in 42MB in a lossless way, why cannot Photoshop do this? \$\endgroup\$ May 16, 2023 at 22:42
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    \$\begingroup\$ @greenberet123, because unlike TIFF/PNG, in RAW files there are no RGB pixels. You can consider it cheating, but the way they count pixels on sensors and thus RAW images is by counting "single-color" cells. // The typical workflow is to save intermediate results in any suitable lossless format and not care about size, and when finished, save the final image in a high-quality JPEG or another lossy format (typically 1/10 to 1/2 size of the source RAW). \$\endgroup\$
    – Zeus
    May 17, 2023 at 0:57

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