1) Make a new Layer, make a selection of the lenses and fill with black. Set the Layer -> Fill to 0%

2) Make sure the sunglasses layer masked out the lenses.

3) Double-click on the Layer with the blacked filled lenses shape to open up the Layer Style.

4) Go to the Gradient section, and double-click on the gradient preview.

5) The new window will let you customize the gradient. Above the gradient bar is a square-down triangle. Left-click that icon, new options will show below and you can change the opacity for that 1-stop in the gradient.

[![Sunglasses][1]][1]


  [1]: https://i.sstatic.net/7MGUh.png