Lens of Truth

The Lens of Truth, also referred to as the Eye of Truth, is an artifact that detects illusions and invisible objects in and.

Ocarina of Time
In, the Lens of Truth can be found in the Bottom of the Well, where Dead Hand guards it. Link first hears about the item 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 shows which walls, floors, and objects are fake, and it allows him to see things such as invisible ladders, platforms, enemies, and Poes. It drains Magic Power when used. 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.

Majora's Mask
In, 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.

Hyrule Warriors
In, a similar item called the Mirror of Truth appears inside the Water Temple area of Lake Hylia. It is used by Sheik to reveal the true identity of Wizzro who is disguised as Princess Zelda.

Additionally, in Adventure Mode, trying to walk onto tiles that have not been unlocked yet will cause an icon resembling a stylized Lens of Truth to appear.

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 Magic.
 * 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.