Problem with merging layers. Image changes.

potamcha, this is on-screen resampling. You must view at 100%, which maps exactly one image pixel to exactly one screen pixel.

Try to visualize this example: a three-by-three pixel checkerboard pattern, crisp black and white, zoomed out on screen, so that there is only one screen pixel to represent it. Right. You get one gray screen pixel.

This is all about pixels. A starry night is like an extremely noisy image. You need to see every pixel to get a true representation. And you do that by viewing at 100%.

Now - this can bite you if you have layers and adjustments. For performance reasons, Photoshop keeps a cache of progressively downscaled versions. This is known as the image pyramid. When you zoom out, Photoshop goes to the cache and retrieves the appropriate smaller version, and puts that up on screen.

But it also uses this cached and smaller version for all the blending previews and adjustment previews. And this is where it bites you, because the small version is not a true pixel representation. It has a lot of intermediate blended values that aren't really there in the original data. This is the checkerboard turning gray. And so you get an incorrect preview.

When you merge the layers, however, the full data are merged. And that may give a different result for each pixel.

Bottom line - always, always, always view at 100% for a reliable preview. It should be in your blood. You should press ctrl+1 by habit, every time you need an accurate preview.

For a "normal" photograph this doesn't matter. But a starry sky isn't a normal image - it's technically pure noise.