Bokeh
Bokeh describes the quality and character of out-of-focus rendering — smooth, busy, ringed, swirly — not the amount of blur, which is what depth of field measures instead. Two lenses can produce identical depth of field at the same focal length, aperture and distance while rendering their out-of-focus regions in completely different ways.
Bokeh comes from factors depth of field doesn't touch: the aperture diaphragm's blade count and shape (more blades, especially rounded ones, generally produce smoother out-of-focus highlights), spherical aberration correction, and specific lens-design choices some manufacturers make deliberately for rendering character rather than pure sharpness. It's a real optical property, but a genuinely separate one from how much of the frame is in focus.
Frequently Asked Questions
Does a wider aperture always give better bokeh?
It gives more blur, which isn't the same as better quality of blur — a wide aperture on a poorly corrected lens can still produce busy, distracting bokeh, while a well-designed lens at a moderate aperture can render smoothly.
What causes 'onion ring' bokeh?
Visible concentric ring texture in out-of-focus highlights, most often traced to certain manufacturing methods used for aspherical lens elements, a specific and identifiable optical-design side effect rather than a universal property of all lenses.