Merge Layers in Photoshop changes the image

[tsinsf]

I have 6 TB of photos and am trying to reduce the data file size of most of my PSD images. The background layer on all is a smart object version of the original CR2 raw file, and then all the layers on top of that are edits. I want to save the Smart Object background layer and consolidate all the other layers. When I select all of the layers on top of the background layer and choose Merge Layers occasionally it changes the photo, always making it lighter. What give, and how do I fix the problem? Thanks!

Photoshop 2021, Mac OS 10.15.7

[Peano]

Have you tried stamping (instead of merging) all layers except the background layer, and then delete everything between the stamped layer and the background? Seems like that would accomplish the same purpose.

[Benny Profane]

Try select all, then copy merged, and paste new merged layer on top. Then delete everything else. Or, better yet, make a new file with the same dimensions as the clipboard after you copy the merged file (this will be an option at the time), and then save that separate file as a .tif.

[john beardsworth]

it changes the photo, always making it lighter

Try zooming in at 1:1 and see if you still get the change.

[digitaldog]

Depending on blend modes, this isn't anything new. Indeed, you MUST view this all at 100% or greater.

[tsinsf]

Thanks for the tip. Is stamping a bunch of layers different than selecting those layers and activating Merge Visible plus pressing the Option key?

[Peano]

Thanks for the tip. Is stamping a bunch of layers different than selecting those layers and activating Merge Visible plus pressing the Option key?

I'm not sure if they're the same.

[john beardsworth]

Thanks for the tip. Is stamping a bunch of layers different than selecting those layers and activating Merge Visible plus pressing the Option key?

They're the same, but what you seem to have hit is a display difference. 1:1 or 100% is accurate. So just try repeating what you did, and then try zooming in 1:1 and doing it again.

[vrkaya]

In my experience, stamping produces a flattened "layer" that does not change the appearance of the image. I can then delete the other layers. I just prefer it since I get the results I expect Every so often the Merging process has changed the image slightly (I really don't know why) which is another reason I like stamping.

Regards,
Ron

[roonsmits]

Am I right to understand you're trying to free up disk space by flatening your files as you describe?

Why not go for a larger disc? Then copy your data to the new disk and keep the current disk as a back up?
GBs are so cheap nowadays.