Lens of Truth

The Lens of Truth is an artifact that lets Link see "the truth" and is found in Ocarina of Time and Majora's Mask.

Ocarina of Time
In Ocarina of Time, the Lens of Truth can be found in the Bottom of the Well. Link first hears about the item when an old man in Kakariko tells him about a man who could see the truth and whose house stood where the well is now. The Lens of Truth allows the user to see things that are not normally visible to the naked eye. It shows which walls, floors, and objects are fake, and it allows the user to see things such as invisible ladders, platforms, enemies, and ghosts. Be warned, as it drains Magic Power over time. However, if the player sets the lens to a button, then taps the button repeatedly, it will not drain magic. The Lens of Truth is completely optional but makes many areas, specifically the Shadow Temple, less challenging.

The Lens of Truth is also referred to as the Eye of Truth and looks very similar to the Sheikah's eye symbol, with the handle as the tear. The Lens of Truth is a companion relic to the Mask of Truth and the Stone of Agony. It is speculated to be part of a set of three objects, including the Mask of Truth and the Stone of Agony, that were created to give their holder untold power in terms of perception of the truth of their surroundings.

Majora's Mask
In Majora's Mask, the Lens of Truth can be found in a cave in the Goron Village of Snowhead. It functions the same way as in its predecessor.

Trivia

 * In Ocarina of Time, if Link plays the Song of Storms and then uses the Lens of Truth, it will not drain magic energy.
 * In the beta of Ocarina of Time, the Spirit Medallion had the effect of the Lens of Truth. This is evidenced by data left in Ocarina of Time pointing the filename of the Spirit Medallion to the effect of the Lens of Truth.
 * In Twilight Princess, Wolf Link's finely-tuned wolf senses perform a function similar to that of the Lens of Truth.