Will a screen protector affect Face ID?

A correctly-cut protector doesn't affect Face ID. The failures that do occur trace to two specific causes, both identifiable.

What Face ID needs

Face ID projects an infrared dot pattern and reads the reflection with an infrared camera. Both the projector and the camera sit in the sensor housing at the top of the screen.

The critical point is that infrared transparency and visible transparency aren't the same property. A material can look perfectly clear to your eye and still attenuate infrared significantly. This is why the failure mode is confusing: the protector looks fine, and unlock has become unreliable.

Cause one: the cutout overlaps the sensor

The most common cause is a protector cut for a slightly different model. A protector for a similar phone in the same range can overlap the sensor array by a millimetre, which is enough to interfere with the projector or the camera.

This is why buying for the exact model matters more than any specification on the box, and why trimmed-down universal sheets are a bad idea on any phone with Face ID.

Cause two: the material attenuates infrared

Cheap film, heavily tinted glass and privacy filters can all reduce infrared transmission. Privacy protectors are the worst offenders because the louvre layer is designed to block off-axis light, and the Face ID projector and camera are offset from each other by design.

The symptom is characteristic: Face ID works in good light and fails in the dark, or becomes slow and inconsistent rather than failing outright. In bright conditions there's enough ambient infrared to compensate; in darkness there isn't.

Diagnosing it

Before assuming the protector, clean the sensor area thoroughly. Adhesive residue and smears across the housing are a common and trivially fixable cause, and they accumulate exactly where your thumb rests when you pick the phone up.

Then test in a dark room. If unlock is reliable in daylight and unreliable in the dark, the material is attenuating infrared and no amount of re-registering your face will compensate.

What to do

Re-registering your face doesn't help here, unlike fingerprint sensors where it usually does — Face ID builds a depth map not matching an image through a layer, so the problem is signal loss instead of a changed reference.

If the protector is the cause, replace it with clear glass cut for your exact model. If you specifically need privacy, accept that Face ID may be less reliable in low light, or use a privacy protector that explicitly states Face ID compatibility. A small number are cut with a wider sensor clearance for this reason.