Refraction 102
The diameter of the fish’s window is fixed by the physics of refraction. If the apical angle of the window were 90 degrees, then the diameter of the window would be exactly twice the depth of the fish. Since it is 97 degrees, the diameter of the window is 2.26 times the depth of the fish, in theory. In practice, it is much closer to the simple 2X the depth. This is because the images of objects above the surface and under 10 degrees above the horizontal are so very strongly compressed as the light bends sharply over the edge of the window that these objects basically become part of the window’s edge–like a border on the window. This border effectively closes the clean, crisp, see-though portion of the window a bit. So if one simply figures that the diameter of the window is about twice the depth, that’s good enough for angling purposes.
Now note carefully, I said the diameter of the window. The front edge of the window, therefore will be the distance of the window’s radius from the fish. If the fish is down two feet, the front edge of the window is two feet upstream from the fish. There that’s easy. More on estimating fish depth in a future post.
Mike Allen casting from a rock on the Siberian River in NZ. NOTE: mountains, rocks in background.
Mike from under water. Where in the heck are the mountains and rocks? They are folded into the edge of the window. Look carefully, you will see them.
