Why does portrait mode cut out bits of hair and glasses?

Real background blur comes from optics: a large lens opening cannot render near and far sharp at once. A phone lens is far too small for that, so the blur is painted on after the fact, and every artefact you see is a failure of the map that decides where to paint.

Step by step

1. Understand what the phone is actually doing.It estimates distance for each part of the frame — using two cameras, a dedicated sensor, or a trained model — then applies increasing blur with estimated distance. The estimate is coarse, so the boundary between subject and background is drawn as a line rather than measured.
2. Give it more light.Depth estimation is a pattern-matching problem and it degrades in the dark exactly as focus does. The same portrait indoors at night and by a window will produce visibly different edge quality.
3. Increase the distance to the background.The bigger the depth difference between subject and background, the easier the decision and the more convincing the result. A person standing against a wall is the hardest case there is.
4. Stand at the distance the mode expects.Most phones ask you to move closer or further away. That prompt exists because the depth estimate is calibrated for a range. Ignoring it is the single most common cause of a bad cut-out.
5. Simplify the outline.Loose flyaway hair, wire spectacle arms, a lattice-back chair and a hand holding a thin object are all edges the map cannot resolve. Tied-back hair and arms held against the body cut out cleanly.
6. Check the result at full size before you rely on it.Errors are invisible on the phone screen and obvious on a laptop or a print. Zoom to the hairline and to any gap between limbs and body before deciding the shot is good.
7. Edit the depth afterwards where you can.Most phones let you reduce the simulated aperture after the shot. Backing the blur off substantially usually looks far more natural than the default, because less blur means less visible error at the boundary.
8. Use the real answer when it matters.Optical background blur — a longer lens, a wider aperture, more distance behind the subject — has no edge artefacts at all because nothing is being cut out. On a phone, the telephoto camera with the subject well forward gets you part of the way there without any simulation.

Tips

• Turn the effect down rather than off. A modest amount of blur hides its own mistakes.
• Portrait mode on objects, food and pets fails more often than on people, because the models are trained mostly on people.
• If the phone offers a lighting effect as well, check the edges twice — those effects amplify the same map.

Common mistakes

• Shooting a portrait against a wall in portrait mode — With no depth difference to detect, the phone has nothing to work with and blurs almost at random. Move the subject a couple of metres forward and the same shot works.
• Judging the cut-out on the phone screen — The screen is small and the preview is often a lower-quality approximation. Halos and chewed hairlines only appear when the file is viewed at full size.

Questions people ask