It as reasonable to assume that the fish can under ideal conditions, see about 30 feet. This would assume that the fish was some 20 to 30 feet deep in a crystal clear pool with bright sunlight. Now, you know that just because the fish can see something doesn't mean that the fish will react to it. The fish will wait until other senses determine the correct course of action. The fish's feeding instincts will take over and the suspected food item must be of the predominate species, having the correct size, shape and color as it enters the fish's strike zone. Once the decision has been made, a trout will still not venture more than a very few feet to take an offering either from nature or from you.
Hearing
The fish's hearing is quite good because sound travels very far and fast under water. The fish will over time learn its environmental sounds and identify them as normal sounds. When the fish hears something that is not within the normal hearing inventory, the fish will be on a heightened level of security. The fish will attempt to use an additional sense to confirm or deny the potential threat.
Lateral Line Sensors
Mr. Fish has another trick up its sleeve by having organs that are called Lateral Sensors, which can pick up the sound wave vibrations. These organs located on each side of the fish and run down the entire side of the body. These sensors are really nothing more than a line of very sensitive nerve fibers that can feel sound vibrations', which can be felt from surprisingly long distances. These sensing organs start just behind the gill opening and run down along both sides of the fish to just before the caudal fin. I know, what you're thinking, the fish already has ears, so why does it need another organ to do the same thing? I can explain better by simply stating that the ears can hear sound, but the Lateral Sensors can feel the vibrations of those sounds. These organs can't hear but they do offer the fish an ability to determine things in its' environment that are either good or bad. In short, the fish can determine that a particular vibration could be a potential food source or that the vibration is alarming or threatening in nature thus urging the fish to seek protection. These organs can alert the fish to anything that moves in the water column. The fish can feel the presence of almost any creature large or small that inhabits its' environment and it does this before either seeing or hearing what's coming.
Sense of Smell
An article published by a marine biologist at Louisiana State University, outlined a study of the olfactory [smell] and gustatory [taste] ability of trout. In short, the sense of smell is a fish's primary instinctive defense mechanism. In fact, a fish's sense of smell is quite remarkable.
A fly angler must be aware that fish, within their watery environment, are better at smelling and tasting, than the bird dog is in the free air environment. You and I have about 5 million olfactory receptors [cells] in our nose that allows us to smell. Some species of dogs have over 200 million of these cells, plus a volume of four times that of our nose. The lowly trout can out perform all humans and most species of dogs, when it comes to the sense of smell. This is why the fish will depend on this sense above all others to effectively survive within its environment.
Fish have two nostrils on each side of their head, like this ( :: ), where the water enters and exits. The olfactory [smell] system is located between the entrance and exit nostrils on each side of its head. Fish also have the ability to taste, with taste buds located on their lips, tongue, and throughout the inside of the mouth.
Another discovery was that different species of fish, respond to different scents. The reason is that the scent receptor sites are different in size, meaning that the fish can only smell odors that have scent molecules that are the same size or smaller than the scent receptors. In other words, there's no magic odor that will stimulate all fish species. This also implies that your fly may give off scent molecules that are larger than the scent receptors in the fish you are trying to catch.
Now that you have a better idea of the importance of using a fly that isn't offensive to the fish, I give the bad news. Humans give off a very offensive amino acid called L-Serene. This scent is transferred to the fly when handled or tied. There are a host of other offensive odors that will force a fish to reject your offering. At the top of the list are insect repellents, followed by, tobacco products, gasoline, human saliva, sun tan lotions and various blockers, fragrant or lubricating soaps, and last but not least, the actual fly. When you consider the variety of materials used to construct a fly, plus the contamination you impart to the fly during the tying process with your own body acids. Although there has never been a study of the resident chemicals and scents built into a fly, you only have to imagine the possibilities.