Focal Length Determines Distortion
The key difference between front and rear cameras is focal length. Front cameras on most smartphones have focal lengths equivalent to 23–28mm on a full-frame camera. Rear cameras, especially in portrait mode, often use lenses equivalent to 52–85mm. This difference matters profoundly for face matching because focal length determines perspective distortion.
At short focal lengths, nearby objects appear disproportionately large relative to distant objects. Your nose is 10–15cm closer to the lens than your ears when taking a close-range selfie. At 23mm, this distance difference creates 20–30% nose width enlargement and corresponding jaw narrowing. At 52mm or longer, the same distance difference is perceptually negligible.
Practical Differences in Results
Close selfies with the front camera can produce celebrity matches that skew toward people with wider noses and narrower jaws, because the selfie distortion makes your face appear to have those proportions. Rear camera photos at arm's length or further produce undistorted proportions and typically more accurate, consistent matches.
If you get surprising results with a selfie, try the same photo with the rear camera and compare. In most cases, the rear camera match will feel more intuitively correct, it shows your face as others actually see you rather than as the wide-angle lens distorts it.