Short · Optics
Why is the background blurry?
Those glowing circles behind a portrait are tiny lights, smeared into discs. See why a lens focuses only one distance, and why closing the aperture shrinks the blur.
Play with this now
Opens Lens Lab. Change the same thing yourself.
Transcript
Those glowing circles in the background? Each one is a single, tiny light. A lens bends each cone of light back to a point, right on the sensor, but only for one distance. Lights further away focus too soon, spread out again, and land as a disc. That disc is the blur. Now close the aperture. The cone narrows, the discs shrink, and the background sharpens. Play with this now at Explainer Tools dot com.