The Magnifying Lens, also known as the Magnifying Glass, is a quest item from The Legend of Zelda: Link's Awakening. In order to obtain it, Link must conduct a Trading Quest. Once Link has obtained the Magnifying Lens, he can take it to the library west of Mabe Village and read Dark Secrets And Mysteries Of Koholint. This book informs him how to navigate the interior of the Wind Fish's Egg in order to reach the final boss of the game, Nightmare. The navigation pattern is different for each playthrough, so the trading quest is necessary for the completion of the game (unless one tries all the possible combinations of patterns).
The Magnifying Lens can also be used to find an invisible Zora in the Animal Village in Link's Awakening DX. When this Zora is discovered, the Photographer will take a picture of Link and it. Also, in Toronbo Shores there is a cave where the Boomerang Dealer resides. He can only be seen with the use of the Magnifying Lens, and he will trade Link the Boomerang.