JPEG & TIFF Saving from PS at Low Quality

Hi

Thanks for those linked images it was useful to confirm the issue.

So, when viewed at 100% zoom there is no on screen difference between your start image, before or after merging and when viewing the jpeg in Photoshop. When zoomed out, there is a difference before and after merging.

That confirms the issue was as described above by myself and D.Fosse. You really must view before and after at 100% zoom. At less than 100% the image preview is not accurate as it is blended using combined and resampled pixels (at the screen zoom level) rather than the individual image pixels.

As an aside, I notice your jpeg is saved with the ProPhoto RGB profile embedded. That is fine for this comparison and for viewing on your own system and in colour managed applications. But if you ever post it on to the web  make sure you export, from the master PSD, a copy converted to sRGB.

Dave

"I have Milky Way shots taken on a Sony A7ii at f/2.8, 30 sec., 6400 ISO."

That's a textbook case. The point about 100% is that this represents one image pixel by exactly one screen pixel. It's the only way to view the pixel data accurately - and with a starry sky, you have a lot of individual pixels.

Any other zoom ratio resamples the image, and any resampling softens and blurs sharp pixel transitions.

What complicates this is that adjustment previews you see on screen are calculated from the screen image. That's done for performance reasons. If that screen image is a soft, resampled version, you get a lot of intermediate values that aren't really there in the original data. And the calculation is done on this false basis. So you get a misleading preview.

If you view the file at 100% when judging the effect of the adjustment, you don't get this misleading preview. You get a true representation of the pixel data and what the adjustment will do to them.

In other words - there's no degrading. You just got a false preview, because you didn't view at 100%.

In a normal photograph the effect is negligible and this isn't an issue. But with very noisy data, or "binary" images where pixels are either "on" or "off" (line art, halftone screen etc) - you always need to judge at 100%.

Are you comparing before and after merging at 100% zoom. It sounds like you are not. When zoomed out, the preview is blended using 8 bit softened layers. On most images that Alex little difference but with tiny points of light, like stars, the difference can be huge.

Always check your image at 100% before merging.

Dave