Focus breathing makes a shot appear to zoom slightly as the focus is racked, which pulls the viewer’s eye to the frame edges instead of the subject. This calculator tells you the exact digital scale to apply in post to cancel that drift, using nothing more than the lens’s effective focal length at its two focus extremes.
How it works
On a fixed sensor the horizontal angle of view is set by the focal length: a longer focal length means a narrower, tighter frame. So if a lens behaves like a 50 mm at infinity but a 47 mm at its minimum focus distance, the infinity end of a pull is the tighter frame and the close end is wider.
To match them, you crop and upscale the wider frame until its angle of view equals the tighter one. The required scale is simply the ratio of the two effective focal lengths:
scale = longer effective focal length ÷ shorter effective focal length
In the 50/47 example that is 50 ÷ 47 ≈ 1.064, a 6.4% framing shift. You would keyframe a 1.064× scale on the wider (close-focus) end down to 1.000× on the infinity end so the edges stop moving.
Resolution cost
Upscaling the wider frame spends resolution. A 1.064× crop on a 3840-pixel-wide 4K source leaves about 3609 usable pixels before upscaling — roughly 6% of linear resolution. The tool reports both the percentage headroom and the remaining 4K width so you know whether to oversample (shoot 6K or 8K and finish in 4K) to keep the corrected shot crisp.
Tips and notes
The order you enter the two focal lengths does not matter; the tool detects which focus state is tighter and tells you which end of the pull to scale. If the scale exceeds about 1.08, the breathing is heavy and you should plan extra resolution headroom in camera. Dedicated cine lenses breathe very little and will report a factor close to 1.0, while many stills lenses breathe noticeably. All calculation runs locally in your browser.