This method ensures you can play with ur effects before committing to your decision:
M (for marquee tool)>draw oval
Q (quick mask)>control I (inverse)
place selection on to a new layer
control back space (with black as back ground layer, or white if you want a white one)
control D to deselect
change opacity for desired effect.
You can also do the above using levels instead of filling with black.
Enjoy