Like Levels and Curves, Fill and Opacity in Photoshop can often cause confusion for people. Here, the two are explained so you'll come away with a far greater understanding of how they differ.
I must admit that when I first started using Photoshop many moons ago I had absolutely no idea how the Opacity slider and the Fill slider differed. Sometimes, I'd use the Opacity slider on an image I was editing; then, the next day, I'd get a little adventurous and give the Fill slider a little tweak. Perhaps in my mind's eye, I was just trying to be nice and give them equal opportunity. In truth, I had no idea what each did, why they did it, or how they were different. But different they most certainly are.
In this video brought to you by The School of Photography, Marc Newton breaks down the nuances of each slider and helps you understand how to use them both effectively. Perhaps the most helpful part of this video tutorial is that it will also help you to understand Blend Modes more clearly. Blend Modes are often overlooked in the post-production process, but they are incredibly powerful and flexible. Indeed, I would go so far as to say that I never edit any images these days without using Blend Modes in some part of my workflow.
Do you have any more helpful hints about using the Opacity and Fill sliders? I'd love to hear from you in the comments below.