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To this end there are three levels of on-site information I am after. The first is actually seeing fish. Sometimes you can’t miss them: they jump or break the surface, or they’re right there in front of you. But normally you have to look. Looking into water takes practice. The surface acts as a partial mirror, which means a lot of interference from reflected light. So I wear polarizing sunglasses to block the worst of this surface glare. Blocking out the sky from my field of view also helps, either with a hand or a peaked hat. This lets my pupils open up, which allows more light, and hence more information, to reach the light-sensitive cells in my retina. I can now see much more detail in the water. But still, in places, the surface is a psychological barrier. This is because our eyes automatically focus on what is most obvious, which may be surface debris or whatever is reflected in the surface. But it’s possible to train our eyes to override this tendency. One of my many short-term jobs was unloading stuff from delivery trucks for a big auto accessories shop. At the back of the shop, there was a two-way mirror, behind which was the manager’s office. This mirror was the old-fashioned type, with vertical strips of clear glass punctuating the silver. Looking at it from the brightly lit shop, customers would see themselves reflected. But if you made your eyes defocus, you would suddenly see into the darker office behind. And once your focus had latched on to something at this deeper level, it was easy to keep it there. (Modern half-silvered mirrors are more difficult.
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Jeremy Wade (How to Think Like a Fish: And Other Lessons from a Lifetime in Angling)