What is focus breathing, and does it affect depth of field?

Focus breathing is a visible shift in field of view as a lens's focus distance changes — a scene appearing to zoom slightly in or out simply from racking focus, with no actual change to the focal length setting. It's a real-world lens-design imperfection, not something the idealized thin-lens depth-of-field formula this site's calculators use predicts or accounts for.

Why It Happens

An idealized thin lens has a single, fixed effective focal length regardless of focus distance, but real lens designs — especially internal-focusing zooms and many modern autofocus primes — shift internal elements to achieve focus, and that internal movement can subtly change the effective focal length along with it. Some lens designs are engineered specifically to minimize this effect (often marketed as 'parfocal' or breathing-corrected for video use); others show it noticeably, particularly at close focus distances.

Why It Matters More for Video Than Stills

A single still photograph never reveals focus breathing, since there's only one focus point in the frame — but a video shot with a focus pull (deliberately shifting focus from one subject to another during a take) can show a distracting, unwanted zoom effect if the lens breathes noticeably, which is exactly why cinema lenses are specifically designed and marketed around minimal breathing, often at real cost to other specifications like size or maximum aperture.

This site's calculators use the idealized thin-lens model, which doesn't include breathing — see the real depth-of-field math it does compute in the DoF & Hyperfocal Calculator.