Does tippet or leader diameter really matter, or is that fly-shop mythology?

It matters, but mostly not for the reason people think. There is little evidence trout "see the line and know," — the real mechanism is drag: thicker tippet is stiffer, catches more current, and pulls the fly across the water at a speed no natural insect drifts at, and a drift feeder rejects that instantly. Thinner tippet also lets small flies move naturally and sink faster. So drop diameter when fish refuse a good pattern in slow, clear water, and go heavier without guilt in fast, broken, or stained water — the fish cannot analyze what it cannot inspect.

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