You can often use techniques (like focus) in a photograph to highlight the parts your want the viewer to look at. When doing a how to article, it's often critical that the reader can see both something specific as well as the big picture. When you can't control the focus in a photo (or you're working with a screenshot) there's an easy little technique you can use called selective darkening. This helps show the reader exactly what piece of the larger picture you're talking about.
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