.......................  ......Fly Fishing Yellowstone National Park


Window of Vision, Conti.
This window of vision is caused by light refraction. Stick the tip of you fly rod
down into the water at an angle. Notice the rod appears to bend at the point it
penetrates the surface of the water. This optical effect is caused by the change
in speed of light as it goes from one transparent medium to another or air to

Sometimes trout will hold just a few inches under the surface where they can
closely inspect their food and at the same time, expend only a small amount of
energy eating. When they are only a few inches deep, the depth of focus only
allows them to see objects that are within a few inches of them. In other words,
when they are holding this shallow, their feeding lane is only a few inches wide. If

a dry fly passes by several inches away, to their left or right, they may not even
see it. On the other hand, if the trout is three or four feet deep, the depth of
focus is much greater and it has a much larger feeding lane. Although trout can
focus in almost every direction at once, they cannot focus on an object that is
three feet from them the same way they can one that is inches away.

When objects on the surface or beyond first appear in the window of vision or
come in view on the outermost edge of the circular window, they appear much
shorter and wider than they actually are. The more they approach the center of
the window of vision, the more they appear like they should. Objects directly
overhead appear exactly as they should. That means that the appearance of
your fly is changing as it comes into the window of vision from being short and
wide to actually looking like the real thing.

Now don’t misunderstand this to mean that since the trout sees a distorted view
of your fly when it enter the window, that its appearance of your fly is not
important because they see the real insects on the surface in the exact same
manner. They too appear short and wide near the perimeter of the circle. So it is
still a fact that the more your fly looks like the real thing, the more the trout are
likely to accept it for the real thing.

Continued tomorrow

Copyright 2009 James Marsh