SUMMARY
Focus breathing is a phenomenon where a lens's angle of view subtly changes as focus is adjusted, causing the image to appear to zoom in or out. While largely imperceptible to still photographers, this effect is highly noticeable and distracting in video, particularly during rack focus shots. The mechanical movement of lens elements to achieve focus alters magnification, leading to a change in the field of view. This is why dedicated cinema lenses, engineered to minimize or eliminate focus breathing through complex optical and mechanical designs, command higher prices than standard photographic lenses.
TAKEAWAYS
Focus breathing is the change in a lens's angle of view when adjusting focus.
It is often invisible to still photographers but distracting in video footage.
Cinema lenses are designed to suppress focus breathing, contributing to their higher cost.
This effect can make techniques like rack focus appear jarring and unstable.