A lens brings just one plane of the world to a perfect point. Turn the focus ring, open and close the iris, and follow the light.
A lens gathers the cone of light leaving each point in the scene and bends it back to a point behind the glass. That only works perfectly for one distance. Every point at that distance forms a flat sheet in the world — the plane of focus.
Light from a nearer point converges behind the sensor; from a farther point, in front of it. Either way the cone is cut before or after its tip, so it lands as a small disc, the circle of confusion. That disc is the blur. The coloured discs on the ground glass show it for the three marked points of each scene: near, middle and far.
Turning the ring slides the glass along the barrel. Close subjects need the glass farther from the sensor, so it moves out; distant ones need it closer. At ∞ the glass sits exactly one focal length (50 mm) from the sensor.
In the cutaway, the copper sections are the focusing group, the part that actually slides; bare aluminium is the fixed barrel it threads into. Glass is shown as crown and flint slices: each cemented doublet pairs the two to cancel colour fringing. The exploded view lifts the housing away so the glass train and iris stand on the optical axis.
The iris sets the width of the cone (A = ƒ / N). Wide open at ƒ/1.4 the cone is fat and the discs are large. At ƒ/16 it narrows to a needle, every disc shrinks and the sharp zone deepens. It costs light: the camera keeps exposure by slowing the shutter.
Discs under about 0.03 mm look like points in a normal print. The distances whose discs stay under that limit form the sharp zone. It's millimetres deep at close range and grows roughly with the square of the distance.
An out-of-focus highlight is a small picture of the aperture itself. Wide open it's round; stopped down it takes the nine-sided shape of the blades. Watch the small lights and glints in each scene.
Chess covers the basics. Alley: night bokeh, and why reflections focus at their own distance. Landscape: why a model looks like a model. Watch: how thin the sharp zone gets up close. Macro: a sharp zone thinner than a petal. Each scene's guided tour ends with its own lesson.
The image on the sensor is inverted. The ground glass shows it that way; the viewfinder and the live-view monitor flip it back, like your camera does.
Scene scale: 1 unit = 2 cm · 50 mm lens · 36 × 24 mm sensor · thin-lens model · keys: ←/→ focus · ↑/↓ aperture · 1–3 subjects · A/C/E lens · R/P/Z/K overlays · V view · T tour · S panel · F full screen