In The Legend of Zelda: Link's Awakening, Link can complete a trading quest for the Magnifying Lens. In order to complete it, Link must trade many different items to people all over Koholint Island. Certain parts of the trading quest must be completed in order for Link to advance in the game. While the rest of it is optional, the Magnifying Lens, while not technically required, makes traversing the Wind Fish's Egg much easier.