Missile Lock On

The Hero is suddenly under attack from an enemy helicopter. Undaunted, he pulls out his surface-to-air missile and trains it on the chopper. He struggles to keep the vehicle in shot as the red crosshairs converge to the sound of increasingly rapid beeping. Finally he gets a solid tone, the cross hairs becomes stuck to the target like glue and the weapon flashes "Lock On". He pulls the trigger and then dives to cover, knowing that the homing missile is now locked on and will unerringly find its target without further help.

At the other end, the pilot is alerted by sirens and flashing lights that he is the victim of a missile lock. Only with a High-Speed Missile Dodge or a flurry of missile countermeasures can he hope to survive.

This trope is common both in films and video games that involve fire-and-forget weapon systems. While not every example will include the converging crosshairs or the display screen, the "Beep.... beep.... beep, beep, beep, beeeeeeeeeeeeeeeeeeep" lock on tone is almost universal, as is the second or two required for the lock on to be acquired. While representation in media may be more or less accurate to real life, most of the associated tropes for both the person aiming and the person being targeted have their basis in real world technology.

Not to be confused with Camera Lock On, which is a Videogame Trope about keeping the camera pointed towards an enemy.

Anime

 * Almost all Macross series feature a sequence of the protagonist trying to line up a shot. Or ten.
 * We also see Area 88 pilots waiting for the tone before launching missiles.
 * Also done in Sentou Yousei Yukikaze. What separates them from the rest of the trope is that the lock-on tone is taken directly off an F-15J fighter jet from a JASDF airbase for added realism.

Films - Live Action

 * Used in Star Wars during the Death Star run. Even though they lock on they can't hit the target, until Luke uses The Force.
 * The first shot didn't hit because it wasn't actually locked on; the beeping merely indicated that the craft had reached the optimum firing position. Same with the targeting system on Darth Vader's custom TIE.
 * Top Gun

Video Games

 * Stinger missiles feature in most of the Metal Gear Solid games and feature the traditional lock-on behavior, with potential targets being marked with a diamond that the crosshairs must be aimed at until it turns red to indicate a lock on.
 * For both the first two Metal Gear Solid games, you get the Stinger in order to fight flying bosses or the eponymous Humongous Mecha. Of course, you can use them on regular guards if you're feeling so inclined.
 * Both games have Stinger specific missions in the VR training add-on missions.
 * Averted in Metal Gear Solid 3 as the game is set in the 1960's and so the only launcher is the dumb-fired RPG-7.
 * The Stinger returns in Metal Gear Solid 4 as well as the Javelin, although that functions with fly-by-wire manual aiming to differentiate it functionally from the Stinger.
 * The boss fight against the helicopter in the Mansion mission of Ghost Squad equips the player with rocket launchers. Each player has a crosshair that will slowly converge on the target the longer it's aimed at, together with the usual rising beeps and lock-on prompt.
 * The Call of Duty : Modern Warfare games both feature missile launchers in both the single player and multiplayer modes. The Javelin includes a targeting screen that highlights potential targets with green boxed that turn red when locked onto. The AT 4 and Stinger missiles have no screen, and use only the lock-on tone to indicate when a target is being acquired and then locked onto. The RPG is dumb-fire only.
 * For the other end of things, when you are controlling the AC-130 killstreak in multiplayer, you will hear the lock-on warning when you're being fired upon by missile-wielding enemy.
 * The move Lock-On in Pokémon is based off of this, enabling the user to (usually) guarantee a hit on the next turn.
 * The Wing Commander series used this a lot. Heat-seekers could only lock on if you were behind the target, whereas with Image-Recognition missiles you just had to point at the target for long enough. Torpedoes were an interesting case; they were anti-capital ship missiles that took a very long time to lock on, even though capital ships were very slow moving and couldn't dodge. The explanation was that the torpedo had to synchronize with the enemy ship's shields in order to punch through.
 * A staple of the Ace Combat series.
 * Being a western-made, Tom Clancy-verse based clone of Ace Combat, HAWX has it too.
 * Very common in the Colony Wars series, and you also have countermeasure beacons when you're the target of enemy missiles, although the reaction time to use them is punishingly low.
 * The Naval Ops series uses this for ship-launched missiles. An AEGIS system allows you to lock on to multiple targets.
 * Grand Theft Auto: San Andreas has this feature on the handheld Stinger missile, The Apache helicopter and the Jump Jet.
 * Halo 2's Rocket Launcher, though it only worked on vehicles. Seems to have been discontinued for 3.
 * And then brought back for air targets in Reach, which also introduced the Plasma Launcher, which does have this for all targets. The Sabre fighter also uses it, but since that took place in space, no idea if it could lock on to land or air targets...
 * All guided missiles in Warhawk take time to lock on, represented by a shrinking box around the target.
 * Subverted in Half-Life 2 where the missile launchers are "locked" by a laser pointer, with the missile following where it goes. It can also cause hilarious events where you can circle the missile around the enemy.
 * After Burner has instant lock-on missile (instead of a beep you hear "FIRE!") Move the cursor over an enemy, launch. Repeat.
 * Resident Evil 5 has the LRT laser guided system. The launcher itself doesn't actually do anything, however targeting the boss it is used against long enough results in a converging crosshair and a beep when lock on is made. Pulling the trigger result in a space satellite firing a powerful condensed beam into the boss.
 * Rockets and normal torpedoes in Battlefront 2 take some time to acquire target lock, but homing missiles have an instant lock-on.
 * In most of the Star Fox series Arwings and Landmaster tanks can lock on and fire homing bombs and charged laser blasts.
 * The homing missiles in the Descent series make a distinctive sound to their target as they home in. Just one pulse of that sound is enough to make most veterans of the game twitch their fingers to instinctively dodge. Even worse in Descent 3's multiplayer, where players could assign pre-recorded audio taunts to keys. Guess how many people made one of their sounds the homing missile lock-on noise?
 * Free Space had this with Aspect-seeking missiles. And bombs.
 * In League of Legends Urgot's Acid Hunter blades normally fire straight forward, but if someone's been struck with his acid they home in on them. Catelyn's ultimate is a sniper shot that can't miss one she's started lining it up, but can be intercepted by another champion.
 * Armored Core series does this too with their missile weapons. This continues until Armored Core 4 where missile lock is only indicated by a visual cue only without the usual audio cue.

Truth in Television

 * For varying types of targeting systems and varying types of missiles, this can be true to varying extents. A rather bad offender of the "Dramatic Lock-On Delay" was the AIM-4 Falcon, an air-to-air missile carried by US Air Force jets in the 1950's and early 1960's. This missile required the pilot to keep the nose aimed at the target for as long as seven seconds... against fast maneuvering supersonic fighters, and due to the missile's requiring the tracker head being cooled with a limited supply of liquid nitrogen, if you failed to get the lock on the first attempt, the missile was rendered useless. Only five air-to-air kills were ever achieved with this weapon during nearly two decades of service, including two years of frontline service in Vietnam.