Sometimes you need to isolate an object from its background, either to set it against an entirely different background or to remove unwanted detail in the background so that it appears like a studio shot.

Make a copy of the original layer.

Make an opacity mask, past in another copy of that layer.

Adjust levels in the opacity mask for very high contrast, so the background turns white and your subject turns black.

Fill in any spots with white or black, depending on what you need.

Fix the edges with your brush blending mode set to Overlay.

Once you've done that, you've essentially isolated your second layer.

Go back to the layer view, and adjust levels until the background is white.

The subject (which is masked) shouldn't be affected.