After reading several times the books The Trout and the Fly by Brian Clarke and John Goddard and In the Ring of the Rise by Vincent Marinaro it is a good reminder for me that a trout's world is vastly different than ours. The books give us insights into what it might be like being a trout looking up into our world and how water, light and refraction all come together to effect a trout's feeding habits. Although we can never put ourselves in a trout's shoes we can at least use some scientific facts to help us better understand these wonderful spotted creatures that challenge our minds and skills every time they refuse our fly patterns and presentations.
A trout's cone of vision is in reference to a trout's ability to see objects on or above the surface of the water. As a trout gets closer to the surface his cone of vision to the outside world decreases, as shown below.
Trout just under the surface may not even be aware of our fly pattern drifting near them. Think of a heavy spinnerfall and the trout are holding just under the surface. It's no wonder we have to almost drift our dry fly right directly over a trout's nose for the fish to even see it. During spinnerfalls we often see trout in very strict feeding lanes sucking in the spinners.
Everything I have read is as light rays enter water they are bent 48.5 degrees at the lowest points near the horizontal plane, meaning near ground level. The light rays become less bent as you move towards a perpendicular line -- straight overhead. This would indicate that trout has a 160 degree view of the world above but it is all compressed into 97 degree view under the water. Outside the 160 degree area the surface film acts as a mirror reflecting the bottom or objects in the water. This makes me think there is a lot of data to process for a fish with a brain the size of a pea. :-)
Note: In their underwater world a trout can see 330 degrees horizontally. The "cone of vision" does not pertain to what trout can see below the surface.
Anything on the surface that is below 10 degree mark is vitually outisde the trout's window of vision and is not visible yet. In other words, while a mayfly nymph is trying to emerge the trout may see the nymph's body hanging below the surface film but not see the wing yet and the nymph's body hanging below the surface may appear as a double image due to the mirror effect of the surface film. See photo below.
If you want to learn more I highly recommend you read the two books I mentioned above. They also help the winter months pass by a little faster.