Glossary
Bubble bokeh
Bubble bokeh is the effect where out-of-focus highlights render as discs with a bright, sharply defined outer ring, resembling soap bubbles.
Also called soap bubble bokeh
How it works
It comes from undercorrected spherical aberration. Light through the outer zones of the lens focuses at a slightly different distance than light through the center, which concentrates energy at the edge of each out-of-focus disc instead of spreading it evenly. The result is a ring brighter than the middle.
Simple designs with few elements show it most, because there is no extra glass correcting the aberration. The Cooke Triplet formula, three elements in three groups, is the classic case, which is why the Meyer-Optik Trioplan 100mm f/2.8 became the reference lens for the look.
Bubble bokeh and swirly bokeh have different causes and can appear together. Swirl is about the shape of highlights being clipped at the edges of the frame; bubbles are about the brightness distribution inside each highlight anywhere in the frame.
Why it matters when buying
Bubble bokeh is busy by nature. It is a strong stylistic choice that suits isolated highlights and can overwhelm a portrait background, so it rewards deliberate framing rather than casual use.