Optics · interactive
Why is the background blurry?
Top: what the camera records. Bottom: the light inside the lens. Both follow the knobs you turn.
1 See
Every glowing circle behind the pawn is one tiny light. Why does the camera turn points into discs?
2 Change
3 Understand
Model notes and sources
Thin-lens model of an 85 mm lens on a full-frame (36 mm) sensor. The photo uses real numbers: the subject pawn sits at the focus distance (1.2 m by default), the other pawns at 2.2 m and 3.6 m, and the string lights 11–14 m away. The blur-disc diameter on the sensor is c = (f/N) · |v − vs| / v, from the lens equation 1/f = 1/d + 1/v.
The ray diagram is schematic and not to scale: it stretches distances so you can see the cones. Real lenses add aberrations, non-circular apertures and diffraction, which softens everything at very small apertures. Depth of field uses a 0.03 mm circle of confusion.
Sources: Circle of confusion; Depth of field; E. Hecht, Optics, thin-lens imaging.
An ExplainerTools Original. Concept inspired by Ryan Sael’s The Plane of Focus.