Lens of Truth

The Lens of Truth, also referred to as the Eye of Truth, is an item in and.

Ocarina of Time
In, the Lens of Truth is found at the Bottom of the Well, where Dead Hand guards it. Link first hears about it when an old man in Kakariko Village tells him about a man who could see the truth and whose house stood where the well is now.

The Lens of Truth allows Link to see things that are not normally visible to the naked eye. It can show fake walls and floors, and invisible objects and enemies. It drains the Magic Meter when used. The Lens of Truth is optional, as it is not required to complete the game.

Majora's Mask
In, the Lens of Truth can be found in the Lone Peak Shrine, a cave east of the Goron Village in Snowhead. The cave can be reached by jumping across invisible platforms, which are revealed by Kaepora Gaebora's feathers. It functions the same way as in Ocarina of Time, but can now be used see the souls of deceased characters, such as Darmani, as well as characters that are totally unnoticeable, such as Shiro.

Hyrule Warriors
In, the Lens of Truth is a relic that allows the wielder to dispel illusions. It was left in The Water Temple by the Hero of Time.

When Impa suspects that the person commanding the Dark Forces during the Era of the Hero of Time is not truly Princess Zelda as she claims, she urges her soldiers to ignore the fake princess's commands to cease their attack. Princess Ruto suggests they use the Lens of Truth, located somewhere in the Water Temple, to prove the alleged Zelda is an impostor. While the Hyrulean Forces encircle the temple, the Zora Princess sneaks into the temple to search for the relic. Ruto easily infiltrates the temple and finds the Lens of Truth, but before she can rejoin with her allies, the fake Zelda closes the Central Hall and traps her. Frantic, Ruto instructs the Hyrulean Forces to open the floodgates inside the temple in order to reach the Gatekeeper holding the doors. Pulling the two floodgate switches found in the temple's southeast and southwest rooms causes the moat surrounding the central chamber to recede, allowing the Hyrulean Forces to slay the Gatekeeper and free the Princess, who hands the Lens of Truth to Sheik before retreating the battlefield. Sheik uses the newly-acquired relic to reveal the hidden chamber, housing the Gate of Souls, from which the impostor is issuing her commands, and the Hyrulean Forces quickly engage her in combat. After wearing down the imposter's stamina, Sheik uses the Lens of Truth to reveal their true identity: Wizzro.

In Adventure Mode, the Lens of Truth is used as the selector icon for tiles that have not yet been unlocked.

Theory
The Lens of Truth is speculated to be part of a set of three objects, including the Mask of Truth and the Stone of Agony, that were created by the Sheikah tribe to give their holder untold power to perceive the truth of their surroundings.

It is also possible that the man who's house stood where the Well is was responsible for creating the Lens of Truth (likely along with the other two aforementioned objects). If this were the case, the man would likely have been a Sheikah.

Trivia

 * In Ocarina of Time, if Link plays the "Song of Storms" and then uses the Lens of Truth, it will not drain his Magic Meter.
 * In a early version 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. This would provide further proof that the Spirit Temple was meant to be completed before the Shadow Temple, since the Lens of Truth is required to navigate the latter.
 * In, the Howling Stone architectural design resembles the Lens of Truth, with both including the Sheikah Eye Symbol.