Hello. After tonal adjustment in Curves or Levels (after pressing OK button) the tones of photo changes (usually it became less contrasty than in preview)
Thanks for help!
You're seeing a false preview because you're not viewing at 100%, and you have a very noisy or "binary" image with sharp pixel transitions.
Mid-tonal adjustments have no effect on pure black (0) or pure white (255). They are still pure black and pure white no matter how you pull the curve.
For performance reasons, adjustment previews are calculated on the basis of on-screen zoom ratio. At other zoom levels than 100% you have a resampled and softened version, which introduces intermediate values not present in the original. This is why the adjustment seems to have an effect in the preview, but not on the full data.
Short version - always check adjustments at 100% view.