Lenses have appeared, in one form or another, throughout the Fatal Frame series as a means of augmenting the Camera Obscura. Lenses can be equipped onto the Camera Obscura and used to inflict more damage or a special status. In general, Lenses must be found or unlocked, and their use is limited by consuming some form of spiritual power or items.
Specialized shots were initially introduced in Fatal Frame as Bonus Functions. Although not physical lenses, they function analogous to the Power-Up Lenses of future games. Only one Bonus Function can be equipped at a time, and each can be used in viewfinder mode by aiming and pressing L1. Each use of a Bonus Function consumes one Spirit Stone. Bonus Functions come inherently within the Camera Obscura, but must be unsealed before being used. They can be unsealed from the camera menu by "paying" an amount of Mystical Power, which Miku can do right after exorcising her first ghost.
Fatal Frame II: Crimson Butterfly
Fatal Frame II is the first in the series to adopt the name Power-Up Lens. Power-Up Lenses can be found all over All God's Village and they can be upgraded with Spirit Orbs and Spirit Points like the camera's other functions, further increasing their potency. Only one lens can be equipped at a time, however, a special function allows for up to three to be equipped and switched between while in viewfinder mode. Spirit Power is required in order to use a lens, with the stronger lenses requiring more spirit power.
FF II Power-Up Lenses
Fatal Frame III: The Tormented
A special lens is known as an Upgraded Lens in Fatal Frame III. They work identically to the Power-Up Lenses of the previous game, with the only differences being the appearance of the lens icon, the Spirit Points they consume and the Photo Points used to upgrade them. Like in Fatal Frame II, the "Switch" function allows for three lenses to be equipped at once. It's also worth noting that only Rei and Kei are able to use Upgraded Lenses.
Fatal Frame IV: Mask of the Lunar Eclipse
Lenses in Fatal Frame IV are known as Kyouka Renzu (強化レンズ) and can be translated as Enhancement Lenses or Power-Up Lenses. Like in previous games, using these lenses consume varying amounts of Spirit Power and up to three can be equipped and switched between once the "Switch" function is found. These lenses are upgraded by using Red Spirit Shards, with higher levels requiring more crystals to be used. Additionally, some lenses are specific to the Camera Obscura, while some are specific to the Spirit Stone Flashlight.