Focus isn’t a place you look.It’s a plane you move.
Wide opening. A sliver of sharpness.
On touch screens, use one finger to orbit and two to pan or pinch.
The selected 24, 50 or 85 mm thin lens, a 36 × 24 mm sensor, and a 0.03 mm acceptable circle of confusion drive every measurement. The near and far limits come from the same blur-circle equation used by the live photograph.
The lens housing and sensor are enlarged to reveal the mechanism. Internal group travel, image-space distances, and blur discs are magnified for visibility. The optical stack represents a compound lens; representative rays bend at its equivalent principal plane.
The landscape is an exhibit-scale diorama. The live photograph applies continuous blur from sensor-plane circle diameter. Its nine-sided bokeh follows the iris; the small background lights make those shapes visible. Exposure is compensated when you stop down, so brightness stays constant. Diffraction and aberrations are omitted; f/16 means deeper acceptable focus, not infinite detail.
The sensor image is inverted. The viewfinder rotates it upright, as a camera does.
Geometric depth of field depends on a viewing-size assumption. The luminous centre is exact focus; the surrounding volume is acceptable sharpness, not a hard edge.