Photoshop: Fill vs. Opacity

Everyone understands the effect of the Opacity control in Photoshop, but directly below the Opacity control lies the fairly misunderstood Fill control. Stated in words, the Opacity control affects the overall opacity of all elements on a layer including layer effects, while the Fill control affects only elements on a layer, leaving layer effects untouched. Lets look at an example. The image above contains a simple layer with several layer effects applied. Lowering the value of the Opacity control produces an expected result:

However, if instead the Fill value is lowered, the layer contents are modified differently:

In this example, only the elliptical shape in the center is effected while the layer effects applied to the layer are unchanged. Understanding Fill values provides another facet of layer control. Specifically it allows using layer contents to control the shape of layer effects; setting the fill value to 0% hides the layer contents while still leaving the layer effects fully visible.