Changes perspective strength. It affects the camera look, not which depth is sharp.
3D focus experiment
A plane can have a focal band.
The card is one tilted 3D surface, but its visible output is sliced into narrow strips. Each strip receives blur according to its distance from a focus plane, creating a near-soft → sharp → far-soft depth-of-field effect.
Not a flat blur overlay.
Each vertical strip gets its own blur from its estimated Z-depth.
Define where sharp focus sits and how much depth remains fully in focus.
A practical DOM approximation until the framework has a shader or true per-pixel depth map.