Almost every authority tells you not to submit a selfie. People assume this is about the outstretched arm being visible, or about composition. It is not. It is about what happens to a face photographed from 50 cm away.
Perspective, not the lens
The effect is usually called lens distortion, but the lens is innocent. What changes the face is distance.
At arm's length your nose is roughly 45 cm from the camera and your ears are roughly 60 cm. That is a ratio of about 1 to 1.33, so the nose is rendered about a third larger, relative to the ears, than it would be from further away.
Step back to two metres and the nose is 195 cm away and the ears 210 cm - a ratio of 1 to 1.08. The difference collapses, and the face is rendered close to how it actually is.
This is why portrait photographers use longer lenses from further back. They are not after compression as an effect; they are after a face that looks like the face.
What that does to the measurements
Face comparison works on relationships between landmarks: the distance between the eyes against the width of the face, the position of the nose against the line of the eyes, the shape of the jaw. Change the apparent proportions and every one of those numbers shifts.
The shift is small. It is also systematic, in a system designed to detect exactly this sort of difference. A selfie does not look wrong to a human, which is why the rule needs explaining.
The other problems, in order
- The front camera is the worse camera. On every phone. Lower resolution, a smaller sensor, and more aggressive processing.
- Beautification is usually on by default for the front camera, and smoothing or reshaping is retouching, which is refused.
- The arm changes the pose. One shoulder rises, the head tilts slightly towards the raised arm, and the body turns a few degrees.
- The gaze goes to the screen. The lens and the preview are centimetres apart, and the difference is visible.
- Camera height drifts. Held slightly high, which is the flattering angle everyone defaults to, and which puts the eye line out of the band.
What to do instead
If someone can hold the camera, have them stand a metre and a half to two metres away at your eye level and use the rear camera.
If you are alone, prop the phone at eye level - a shelf, a stack of books, a small tripod - step back to the same distance, and use the timer or a voice shutter. This is a genuine solution rather than a compromise: you get the rear camera, the correct distance, a level pose and a natural gaze.
Framing from further away leaves the head smaller in the frame, which is fine. Crop in afterwards. That is what the resolution is for.
IDShot shows the document frame and head target in the viewfinder and fires automatically when the framing is right, which is what makes the propped-phone approach practical - you are not reaching for a button at the moment of capture.