I've seen this topic floating around but none of the work arounds are working for me.
Working in RGB, I'm just trying to crunch the levels on a layer filled with noise. so i can get some nice little specs.
I apply the noise, select levels get it to look like i want hit ok, then nothing.
I've tried viewing at 100%, same result.
I've tried a new document. Same DPI (300)
Ive tried a new doc, at 72 Dpi.
I've tried using an Image.
I've tried it in a layer mask
I've reset the preferences on PS
I've restarted the computer.
All the same result.
I did get it to work on a Adjustment Layer but when I tried to flatten the group, the levels went away.
Losing my mind over here. I use this trick all the time. or at least i thought I did.
Always view at 100% for an accurate preview. What you're seeing in the preview is an artifact, which happens with very noisy images, or "binary" images such as line art or halftone screens.
The adjustment preview is calculated on the basis of on-screen zoom ratio. This is for performance reasons.
For any other zoom ratio than 100%, that means a scaled, and therefore softened version. This introduces intermediate values where the original has sharp, pixel-level transitions.
Keep in mind that mid-tonal adjustments - like the middle slider in Levels - have no effect on values that are close to 0 (black) or 255 (white). They only work on midtones, with diminishing effect as you go towards the endpoints.
When you flatten, or commit the adjustment, the calculation is performed on the full image data, and so the adjustment you saw in the preview seems to disappear. But in fact it wasn't real and never there in the first place.
So - make it a habit to always hit ctrl+1 to check at 100%. This maps one image pixel to exactly one screen pixel.