Updated: May 7, 2020
Previously we've declared that trout see colors and tippets differently than humans. When delivering a fly it's all about the presentation, to get the fly to behave as natural as possible. Here's the next episode where I'll cover their split vision.
The first and most obvious distinction between the trout eye and the human eye is the shape. While human eyes have a spherical shape the trout have a elliptical shape. This basically means that human have one focal length while the trout have two simultaneous focal lengths. Trout can in other words see in front and to the side at the same time. This split vision affects every fly fishermen's approach to the water.
Trout always have fear of aerial predators such as birds as well as predators in the water. They don't at all like sudden shadows or objects over their head. When approaching a feeding trout we need to sneak up without being detected which may be quite a challenge. The lateral field of vision is farsighted to detect threats while the forward looking field is nearsighted which helps them feeding. I remember one time I was fishing in Hemsila river in Hemsedal, Norway. That's without a doubt the most challenging water I've ever fished, crystal clear water, low water at the time and trout that have seen an artificial fly more times than I can count. I was at my sneakiest approach ever but every time I tried to get in position for doing my cast I could see the fish move away a couple meters and continued feeding. Sometimes a stealthy approach when wading isn't enough and it tells that fish can detect you from far away. But still, if fishing was easy I would have left the sport long time ago.
An angler can take advantage of the blind spot at about 30 degrees behind the fish when faced upstream. However, a slight turn from the fish may reveal your presence. But even if a trout will turn to examine objects they do have limited independent movement within the sockets. When looking for trout I always try to sneak in from behind, and if doing it right I can be able to come quite close to the fish. And if it's possible I stay away from wading the water.
I hope you've found these articles helpful and don't hesitate to comment your thoughts.
Until then, take care and tight lines!