Aim down sights

Aiming down sights or iron sights, is a gameplay mechanic introduced into the Fallout series in Fallout: New Vegas and appeared in Fallout 4 and Fallout 76. Fallout 3 does not have true iron sights, but has a simple zoom alongside the weapon (sans weapons with scopes).

Overview
When aiming down the sights, the camera shifts to behind the weapon's sights. This generally makes the weapon more accurate by eliminating most weapon sway, and may provide some zoom for more usability at longer ranges.

Fallout: New Vegas
Usable iron sights were added on to nearly all weapons. Iron sights can vary wildly between weapons, with some weapons even having damaged sights. When aiming down sights, weapon spread is decreased by using the weapon's iron sights and when certain conditions are met, such as Strength or skill requirements, and the movement and stance done when firing. The full list of penalties:

Penalties to accuracy
These variables specifically affect player's spread. Total spread is then calculated against the weapon's spread. A weapon's spread is completely static and based on the weapon's "Min Spread" value.

Weapon spread variables

 * Weapon mods with Decrease Spread effects
 * Ammo Effects that alter spread

Perks

 * Weapon Handling

Fallout 4
Aiming down the sights is the basic way to improve accuracy with firearms, whether they use iron sights, reflex sights or scopes. Certain weapon modifications increase accuracy while aiming down the sights or firing from the hip, however, these weapon mods will also take a toll on accuracy when not aiming as the mods are intended. Scoped weapons will have noticeable sway when being used, but armor mods for arms have stabilizing modifications for mitigating weapon sway on scoped weapons.

Perks

 * Commando
 * Heavy Gunner
 * Steady Aim

Perks

 * Crack Shot
 * Ground Pounder
 * Long Shot
 * Modern Renegade
 * Skeet Shooter
 * Sniper
 * Stabilized

Behind the scenes
Fallout: New Vegas is the only game with the option to "turn off iron sights."