Sky Replacment: Issues with scaling of sky

[MarkusPaul]

When using the sky replacement tool for astronomy photos where the foreground is taken with different exposure time versus the sky but everything else is identical (same resolution, dpi, images taking from exactly the same vantage point), PS somehow scales up the sky image for no apparent reason. The sky image can then be scaled down again in sky replacement tool but this adjustment is not fine enough and should not be necessary since the foreground and background fit exactly on top of each other (tested by putting both into layers and blending them together). Is there a way to prevent PS from scaling up the sky image or is this perhaps a bug? Image of foreground and sky in the below screen have same resolution & dpi, yet when scale is set to 100 the sky image is larger than the foreground image as is visible from the empty space around the rock in the center.

[D Fosse · Accepted solution]

I think replace sky will automatically do all kinds of scaling. Remember - there's normally no need to preserve any detail in the sky, because there won't be any. So scaling is not a problem.

In other words, I think you're using the wrong tool for this job. This isn't the intended use for sky replacement.

Not having your originals, I'd still think this can be done with Blend modes Lighten or Darken, or possibly Blend If. You may not even need to mask anything other than a very loose mask.

If you do need to mask, check the individual channels. The blue and/or red channels will probably separate this well.

[MarkusPaul]

Thank you very much for the suggestion. I ended putting the two images into layers, using a layer mask and the "select and mask" tool to smoothen the transition between the two images and this worked well.