Magnifying-Glass Damage, formerly known as Hoop Damage, is the damage that a fighter receives when they are out of the camera's boundaries but not past the blast line. It is present in all games except the original Super Smash Bros.
When a fighter is off-screen but not past the blast line, they are still shown inside a small hoop known as the Magnifying Glass at the edge of the camera boundary. Magnifying-Glass Damage is damage applied while the fighter is displayed this way. It is applied at a rate of 1% per second, but stops occurring when the fighter has accumulated 150% damage. In Ultimate, the size of the magnifying glass is determined by how close a player is to a blast line: the closer to a blast line, the smaller the glass gets. The edges of the glass flash yellow when the player is dangerously close to the blast line.
Magnifying-Glass Damage does not occur in Training Mode nor in the All-Star Rest Area in Melee and Brawl. In The Subspace Emissary, if Player 2 is in a situation that would usually trigger the effect, they instead usually Space Jump. Charizard, Meta Knight, Kirby, and to a lesser extent, King K. Rool, all have up throws that send them above the upper blast line with the accompanying Magnifying Glass, but they do not take damage as a result.