What is focus breathing?

Focus breathing is the change in angle of view that many lenses show as they focus from infinity to close range. As the lens elements move, the effective focal length shifts, so the framing zooms in or out slightly during a focus pull, which is distracting in video.

How is the effective focal length estimated?

Breathing is strongest at minimum focus and zero at infinity. The tool interpolates between these endpoints using the focus distance, converting your measured breathing percentage into an effective focal length at the chosen distance. The model assumes breathing scales with magnification.

What does the compensation factor mean?

It is the crop or digital zoom you apply in post to cancel the framing change. If breathing widens the view at close focus, you crop in by that factor so the subject stays the same size across the whole pull, producing a clean breathing-free shot.

How do I measure my lens breathing percentage?

Frame a ruler or wall at infinity, note the field of view, then refocus to the minimum distance without moving the camera and note the new field of view. The percentage change in the captured width is the breathing figure to enter. Many lenses breathe 5 to 15 percent.

Do modern lenses still breathe?

Many do, though some recent cinema and hybrid lenses are designed to be breathing-free using internal compensation, and some cameras apply electronic breathing correction. For uncorrected stills lenses used in video, this calculator helps you plan the post crop in advance.

Focus Breathing Compensation Calculator

Focus breathing is the quiet enemy of clean focus pulls: as the lens racks from far to near, the framing subtly zooms, pulling attention away from the subject. This calculator models how much the effective focal length shifts at any focus distance so you can pre-plan the crop that cancels it in post.

How it works

Focus breathing is zero at infinity focus and greatest at the minimum focus distance. The breathing figure you measure is the angle-of-view change between those two extremes. The tool interpolates the effective focal length at an intermediate focus distance.

It uses a magnification-based model. Magnification at a given focus distance D for a lens of focal length F is approximately:

m ≈ F ÷ (D − F)

Breathing scales with how close you are to minimum focus, so the effective focal length at distance D is:

F_eff = F_nominal × (1 − breathing% × (m / m_min))

where m_min is the magnification at the lens minimum focus distance — the point where the full measured breathing applies. At infinity, m is near zero and F_eff returns to the nominal value.

The compensation factor

To keep framing constant, you crop in post by the ratio of the nominal to the effective focal length:

crop factor = F_nominal ÷ F_eff

A crop factor above 1 means you zoom in to restore the wider breathing view; a factor below 1 means the view narrowed and you would need to zoom out (only possible if you shot with headroom).

Worked example

A 50 mm lens that breathes 10% reaches roughly 45 mm effective at its closest focus. Pulling focus from a distant subject to a close one therefore widens the shot. To hold framing you crop the close-focus frames by about 50 ÷ 45 ≈ 1.11×, or 11%, ramping the crop in sync with the focus pull.

Notes and tips

Shoot with headroom. Frame slightly wider than final so you have room to crop in without running out of pixels.
Measure once per lens. Breathing is a fixed optical trait; record the percentage for each lens and reuse it.
Some bodies correct electronically — check whether in-camera breathing compensation is active before adding a post crop, or you will double up.

All calculations run locally in your browser.