The Trout's 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

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.
Copyright 2013 James Marsh
100% Satisfaction Guaranteed
Free Shipping Continental U. S.