From Liquipedia Starcraft 2 Wiki
The Photon Cannon (often shortened to just Cannon) is the sole static defense building for the Protoss. The player must control a Forge before building Photon Cannons. It's more versatile than its Terran and Zerg counterparts as it can attack both air and ground units. Photon Cannons also serve as detectors with an extremely large vision range.
Like most Protoss structures, the Photon Cannon can only be warped in at a location powered by the Psionic Matrix. It can attack and serve as a detector only while it remains within the Matrix, otherwise it becomes unpowered and cannot function.
 Competitive Usage
When placing a Photon Cannon, a white ring appears along with the green ghost outline of the cannnon. This ring indicates firing range only. The visual and detection range of the cannon is nearly 60% larger than the firing range. If you simply need detection around your mineral line or other area and not necessarily the attack capabilities a single cannon can easily supply vision for the mineral line against ranged attackers such as Ghosts and Banshees.
Players typically will build Cannons in or around their mineral line to protect against (or, belatedly, to react to) harassment. In late stages of the game, expansion bases that the player deems difficult to readily defend will often be bolstered by a few Cannons as well.
Photon Cannons are used offensively in the relatively common Photon Cannon rush technique. Cannons deployed outside the player's own base can also be used offensively after the early stages of the game to deny or delay expansions.
 Vs. Protoss
Photon Cannons can be used to detect and therefore limit the effectiveness of Dark Templar as well as defend expansions later in the game as harass becomes more common.
 Vs. Terran
Photon cannons can be used as a fallback detector if your Observers get picked off. This is critical as losing sight of cloaked Banshees can easily cost you the game. Protoss has no other detection methods.
In some cases Photon Cannons may be a more effective mineral sink than investing in Zealots, especially on defense. For example, Photon Cannons can take advantage of shooting from high ground, while you Force Field a ramp.
The range of the Ghost's Tac Nuke Strike is 10, while the vision range of the cannon is 11. The cannons can act as a good deterrent to nukes striking much deeper than your cannons for the cost of minerals only.
 Vs. Zerg
Photon Cannons are useful in any build, especially in the Fast Expansion against Zerg.
The Photon Cannon can be used offensively in the beginning of the game. By placing two pylons at the bottom the opponents ramp you can then place a Photon Cannon behind your Pylons. This can be used to:
- Force the Zerg player to build Roaches, Banelings or Spine Crawlers to break the contain.
- Shut down early expand builds such at 14 Hatch 14 Pool and similar.
Cannons can also be used to help defend against Mutalisk harass, holding the mutas at bay until your stalkers can arrive.
 Patch Changes
|Patch 7 (version 0.9.0.14621B)|