Sight is of crucial important to trout. After all, it’s the sense that they use when determining whether or not to eat something that comes their way. Trout, in particular, have excellent close-range vision although they lack in long-range vision. This close range vision is why so many imitation flies fail to grab the interest of a trout. A trout sees the world through what is known as the “trout’s window”. This window is a cone shaped view that extends up from the eye at an ever-increasing diameter. Thus the deeper the trout is, the more the trout can see.
A trout will only eat something that passes within this cone of vision since this is where they can see it and inspect it. Due to their other excellent senses, trout may very well be aware of something on the water that is outside of this cone of vision. But a trout consider eating it if it sees the fly in their cone of vision.
This knowledge leads to one strategy all anglers should use when fishing for a rising trout. When fly fishing for a rising trout, it is very important to drop the fly not where the rise was but upstream of where the rise was. By presenting a fly upstream from the rise, the fly will float down the river naturally, not just suddenly appearing in the trout’s cone of vision, which is likely to seem suspicious to a cunning trout.
Trout also have the ability to determine color, including subtle shades of color. This is why the same fly in two different colors can produce remarkably different results when fishing; the trout may simply be eating one color insect and avoiding others.
Lastly, trout also have the ability to see the profile of a fly. Of all the characteristics of a fly, this is perhaps the most important. A fly that does not have the same profile as seen from underwater compared to what they are accustomed to eating, is not likely to be very successful. For this reason, it is always important to make sure that your flies float properly, especially when dry fly fishing.
Moreover, even if your dry fly is a spitting imitation of the real thing, it is not likely to draw strikes if your fly floats awkwardly (or partially sinks).
This is a good reason to avoid the cheap flies you may see – the colors of these cheap flies may look right, but the profile is likely to be wrong when on the water.