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


Choosing the Right Fly - Part 6

Binocular and Peripheral Vision:
When us humans look ahead, our field of vision allows us to see thing that are
within a 176 degrees area called the “field of vision”. Our forward zone of
binocular vision is 90 ninety degrees or forty-five degrees on either side of a
line straight ahead. The portion of our vision that is outside of that 90 degrees
zone of binocular vision represents the area of our peripheral vision. Our
peripheral vision represents a total of 86 degrees or 43 degrees on each side of
our binocular vision.

To illustrate this, place your finger about two feet directly in front of your face
and focus on it. Now continue to look directly forward and move your finger to
your left until you cannot clearly see it in focus. You should be able to see it
clearly until it is 45 degrees left or right of straight ahead. The area in which you
are able to see it clearly is the area of your binocular vision. If you continue to
move it left up to 15 more degrees you should still be able to see the finger but
your cannot see it clearly or in focus. This area represents the area of your
peripheral vision. Of course things work the same if you move your finger to the

Trout have a much narrower width of binocular vision than humans. The trout’s
binocular vision allows them to only focus on things that are within a total of 30
thirty degrees directly ahead or fifteen degrees on either side of a line directly
forward of their eyes.  However, they have a much larger field of vision than us
humans. It is a total of 330 degrees or represents an area almost completely
around them. Of that 330 degrees field of vision, their zone of peripheral vision
represents 300 degrees of it or 150 degrees on either side of their narrow 30
degrees binocular zone.

When they detect something with their peripheral vision, they must move their
eyes towards the object in order to focus on it. There is only an area of 30
degrees directly behind a trout that is not visible to them. This narrow area is
commonly referred to as their blind zone. The bottom line to this is that although
trout can detect movement and contrast almost all the way around themselves
they must look almost directly at an object, or align the object in their narrow 30
degrees field of binocular vision, in order to clearly see it.

Continued tomorrow

Copyright 2009 James Marsh