What's the difference between Fill and Opacity? (Photoshop CS2/CS3/CS4/CS5)

Have you ever wondered what the Fill option on the Layers panel is for? It seemingly does the same thing as the Opacity option just above it, which is to control a working layer's level of transparency, but there's a little more to it. When you set the Opacity value for a selected layer, the change applies to the entire layer. This means that any layer styles and the blending mode are also affected. When you set the Fill value, however, only painted pixels and vector shapes are affected.

For example, when we set the Fill value for a selected shape layer to 66% and set the blending mode to Overlay, only the fill pixels inside the shape are affected. The black Stroke effect on the same layer isn't.

To set the Opacity and Fill values for a selected layer (other than the background layer) via the Layers panel, do one of the following:

• Use the Opacity and Fill sliders to set the percent of opacity.
• Enter a value in the Opacity and Fill text boxes.
• Hover your cursor over the option's name and click and drag the resulting icon right to increase the value or left to decrease the value.
• Select the Move tool and then type a number on the keyboard.
• Click the Add A Layer Style button and choose Blending Options from the resulting pop-up menu. In the Layer Style dialog box, use the General Blending options to adjust the blend mode and opacity for the entire layer; or use the Advanced Blending options to adjust the fill opacity for specified items.