Loupe

a magnifying glass over a printed harbour map. drag it to read the print, press the map to bring it there, and raise or lower it to change how much it shows.

key insight: the glass is held at a height, never set to a magnification. a lens at u of its focal length shows the print 1 / (1 - u) times its size, and the bright spot it focuses on the paper is its radius times 1 - u, so the view grows as the spot tightens.

the map is drawn once at up to seven times the stage's size and read through mipmaps. its border is a line of microprint that only the glass at full height resolves.