You can avoid spooking trout in streams by understanding how they see. Trout view the world above the stream through a circular window directly overhead. Think of a cone of vision extending upward from the fish’s eyes. Outside the edges of this cone, reflection obscures the trout’s vision. The diameter of this window depends on how deep the fish lies–it’s approximately twice as wide as the fish is deep in the water. That means you can approach fish in shallow water more closely than deeper fish. And you can estimate how close you may get to any fish and still stay hidden.

To calculate, you’ll need to remember a bit of high school physics. Light rays entering the stream are bent, which extends the cone of vision to a wider diameter above the water surface. To stay completely out of the trout’s view, you’ll need to hunker down under a line drawn at a 10-degree angle from the edge of the sight window.

Illustration by Dan Marsiglio