{"kind":"explainer","version":1,"id":"original-lens-lab","title":"Why is the background blurry?","summary":"Close the aperture and watch the background sharpen. Follow the light inside the lens to see why each far light lands as a disc.","page":{"path":"/explainers/original-lens-lab/","url":"https://explainertools.com/explainers/original-lens-lab/"},"resource":"https://explainertools.com/agents/explainers/original-lens-lab.json","subject":{"name":"Science","path":"/subjects/science/"},"topics":["optics","photography","cameras","aperture"],"format":{"id":"interactive","label":"Interactive demo","resource":"interactive demo"},"creator":{"id":"explainertools","name":"ExplainerTools","path":"/creators/explainertools/","url":"https://explainertools.com/creators/explainertools/","owned":true},"source":null,"tool":{"path":"/originals/lens-lab/","url":"https://explainertools.com/originals/lens-lab/"},"runs":{"mode":"embedded","phones":"supported","label":"Plays here"},"dates":{"added":"2026-10-09","published":"2026-10-09","events":[]},"lesson":{"question":"Why is the background blurry?","eyebrow":"Optics · interactive","see":"Every glowing circle behind the pawn is one tiny light. Why does the camera turn points into discs?","stage":"A photo of chess pawns in front of string lights, above a side-view ray diagram of a camera lens. Light from a far light focuses before the sensor and lands as a blur disc.","caption":"Top: what the camera records. Bottom: the light inside the lens. Both follow the knobs you turn.","controls":[{"label":"Aperture","ends":["wide open","narrow"]},{"label":"Focus distance","ends":["near","far"]}],"presets":["Portrait · f/1.8","Landscape · f/16","Focus on the lights"],"modelTags":["Model · diagram not to scale"],"notes":[{"text":"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.","html":"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 − v<sub>s</sub>| / v, from the lens equation 1/f = 1/d + 1/v."},{"text":"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.","html":"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."},{"text":"Sources: Circle of confusion; Depth of field; E. Hecht, Optics, thin-lens imaging.","html":"Sources: <a href=\"https://en.wikipedia.org/wiki/Circle_of_confusion\" target=\"_blank\" rel=\"noopener noreferrer\">Circle of confusion</a>; <a href=\"https://en.wikipedia.org/wiki/Depth_of_field\" target=\"_blank\" rel=\"noopener noreferrer\">Depth of field</a>; E. Hecht, <i>Optics</i>, thin-lens imaging."}],"sources":[{"label":"Circle of confusion","url":"https://en.wikipedia.org/wiki/Circle_of_confusion"},{"label":"Depth of field","url":"https://en.wikipedia.org/wiki/Depth_of_field"}],"credit":{"text":"An ExplainerTools Original. Concept inspired by Ryan Sael’s The Plane of Focus.","html":"An ExplainerTools Original. Concept inspired by Ryan Sael’s <a href=\"https://sael.net/plane-of-focus/\" target=\"_blank\" rel=\"noopener noreferrer\">The Plane of Focus</a>."},"provenance":"public/originals/lens-lab/index.html"},"media":[{"kind":"short","id":"background-blur","title":"Why is the background blurry?","path":"/videos/background-blur/","url":"https://explainertools.com/videos/background-blur/","seconds":32,"modelLimit":"Thin-lens model of an 85 mm lens on a full-frame sensor, focused at 1.2 m, with the lights about 12 m away. The ray diagram is not to scale.","credit":"Concept inspired by Ryan Sael's The Plane of Focus (sael.net). All visuals are original ExplainerTools work."}],"guides":[],"journal":[{"title":"What does a camera do when it focuses?","path":"/blog/how-camera-focus-works/","url":"https://explainertools.com/blog/how-camera-focus-works/","description":"Follow a few rays of light to understand sharp focus, blur, and aperture, then explore a camera lens together."}],"related":[{"id":"cameras-and-lenses","title":"Cameras and Lenses","path":"/explainers/cameras-and-lenses/","url":"https://explainertools.com/explainers/cameras-and-lenses/","subject":"Science","format":"Interactive article","creatorId":"bartosz-ciechanowski","creator":"Bartosz Ciechanowski"},{"id":"plane-of-focus","title":"The Plane of Focus","path":"/explainers/plane-of-focus/","url":"https://explainertools.com/explainers/plane-of-focus/","subject":"Science","format":"Interactive demo","creatorId":"ryan-sael","creator":"Ryan Sael"},{"id":"original-rolling-shutter","title":"Why do propellers look bent on camera?","path":"/explainers/original-rolling-shutter/","url":"https://explainertools.com/explainers/original-rolling-shutter/","subject":"Science","format":"Interactive demo","creatorId":"explainertools","creator":"ExplainerTools"},{"id":"lights-and-shadows","title":"Lights and Shadows","path":"/explainers/lights-and-shadows/","url":"https://explainertools.com/explainers/lights-and-shadows/","subject":"Science","format":"Interactive article","creatorId":"bartosz-ciechanowski","creator":"Bartosz Ciechanowski"}],"coverage":{"level":"model-notes","note":"The question, controls, model tags, notes and credit come from the tool’s own page."},"provenance":["public/catalog.json","public/originals/lens-lab/index.html","scripts/original-shorts.json","public/content.json"]}