Lens of Truth

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

Location
In Ocarina of Time, the Lens of Truth can be found in the Bottom of the Well, where Dead Hand guards it. The item can be gained a few minutes after the young hero enters the mini-dungeon if he pays attention to where he is going, as the rest of the dungeon is optional. 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.

Uses
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 Lens of Truth is set to a button, which is then tapped 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 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.

It also allows Link to see the Poe guide in the Haunted Wasteland that takes travellers to the Spirit Temple.

Location and Uses
In Majora's Mask, the Lens of Truth can be found in Lone Peak Shrine, a cave east of the Goron Village in Snowhead. The cave can be reached by jumping across invisible platforms, the positions of which are revealed to Link by Kaepora Gaebora. It functions the same way as its predecessor, but is also utilized in new capacities; it can now be used see the souls of deceased characters, such as Darmani, as well as characters that are totally unnoticeable, such as Shiro.

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 in terms of perception of 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 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. 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 Twilight Princess, Wolf Link's finely-tuned senses perform a function similar to that of the Lens of Truth.
 * Also in the Twilight Princess, the Howling Stone architectural designs matches heavily similarities to the item of the Lens of Truth, let alone the Sheikah Eye Symbol.
 * The Magnifying Lens, an artifact found in, is very similar to the Lens of Truth, as it is also a magnifying glass that can reveal hidden objects.