F-Stop was the early Portal 2 mechanic Valve developed before restoring portals to the sequel.
For roughly five months, Valve explored making a sequel that did not initially rely on the portal gun. F-Stop centered on an Aperture Camera that could photograph objects, store them, and reproduce them with changes such as rotation or scale. The prototype was set in Aperture’s 1980s history and followed a different test subject rather than Chell.
Playtesters enjoyed the mechanic but were disappointed that portals were absent. That feedback led Valve to reconsider the project’s direction and bring portal-based puzzles back. F-Stop was never released as Portal 2’s gameplay system, although details about it became public years later and its code was reportedly used in another project. The gels and other mechanics eventually added to Portal 2 complemented, rather than replaced, portals.