Focal length and distortion: what it is
Quick facts
- Definition
- Distance between the optical center of the lens and the sensor, in millimeters, which sets the angle of view.
- Effect on a face photo
- Short lenses, used up close, widen the nose and distort proportions.
- What is usually preferred
- Short telephoto lenses, with more distance between the device and the patient.
- Phone
- The main camera can distort; stepping back and zooming in helps.
- Golden rule
- Same distance and same lens for the before and the after.
Focal length is the distance, in millimeters, between the optical center of the lens and the camera sensor. It sets the angle of view: short lenses see more of the scene and "stretch" the perspective; longer lenses compress the perspective and render the face more faithfully. In a clinical facial harmonization photo, this determines whether the patient's proportions appear reliably.
Why does focal length distort the face?
The real distortion does not come from the lens itself, but from the distance between the camera and the face. A short lens forces you to move close to frame the shot, and whatever is closest to the camera grows relative to the rest. The nose looks wider, the forehead more receding, and the midface more projected.
This hinders the assessment of symmetry and the tracking of results, because a difference in distance between two photos looks like a change in the face.
How do you avoid it in routine practice?
Many clinics prefer to step back and use a short telephoto lens, rather than moving close with a wide-angle lens. On a phone, the rule is the same: standardize. The depth of field also changes with distance and aperture. For the rest of the standardization, see standardized clinical photography.
Relationship to facial measurements
Measurements such as the interpupillary distance and the assessment of facial asymmetry are only reliable in photos without perspective distortion.
In Harmy
The app asks for 1 frontal photo. Photographing straight on, at eye level, and with a constant distance across patients and sessions improves the reading of the face and the comparison with the actual result.
Also called: perspective and lens distortion.
Frequently asked questions
Why does the nose look bigger in a selfie?
Because the camera is very close to the face and the lens is short. What is closer to the lens, like the nose, appears disproportionately larger relative to the ears and the outline.
Can I photograph the patient with a phone?
Yes, as long as the distance and the camera are always the same, with the device at eye level. Consistency matters more than the equipment.