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


Choosing the Right Fly - Part 3

Window of Vision:
You may hear anglers say “the trout were not looking up today”. I assume they
mean that as a figure of speech because trout are always looking up. Unlike
humans, they see almost all the way around themselves. Also, unlike humans,
trout can focus at extremely close ranges. They can focus on a fly that is only
an inch or less from their eyes. However, at long ranges they cannot focus well
enough to discern the details of objects in the water.

Without going into unnecessary detail regarding the physics of light, lets look at
some facts that affect the trout’s vision of your fly. The “window of vision” as it is
called, is the area of water on the surface above a trout where they can clearly
see objects. Trout can see objects on the surface that are directly above them.
If the surface of the water is smooth or not rough, they can see objects directly
above them that are above the water. However, there is a point above them at
which their line of sight will not pass through the surface of the water. It is exactly
48.5 degrees from a point at which a vertical line extends from a trout’s eyes to
the surface of the water.  

This means that they can see through the surface of the water in an area
formed by a 97 degrees cone. This cone looks like an upside down snow cone
cup with the point of the snow cone extending from the trout’s eyes up to the
surface of the water. Using this analogy, if the circle of the cone (or top of the
upside down snow cone) was even with the surface of the water, it would be
referred to as the window of vision.

The trout sees everything that is outside of that cone as a mirror image of the
underwater surroundings. The deeper the trout, the larger the window of vision
is at the surface of the water. If the trout is only a couple of inches deep, the
window of vision is just over four and one-half inches in diameter. If the trout is
two feet deep, then the diameter of the window of vision is just over four and
one-half feet in diameter. In other words the trout can see objects at the surface
of the water just over two and one-half feet in front of, two and one-half feet
behind and two and one-half feet on either side of their position. A fly on the
surface of the water passing over the trout can only be seen by the trout
for a total distance of four and one-half feet or the diameter of its window of

Continued tomorrow

Copyright 2009 James Marsh