Depth of field: what it is and how it affects clinical photos
Quick facts
- Definition
- Range of distances, from the camera backward, in which the image appears acceptably sharp.
- Depends on
- Aperture, focal length of the lens and distance between camera and subject.
- Practical effect
- Shallow depth leaves the nose or ears blurred; greater depth keeps the whole face sharp.
- Relevance in facial harmonization
- Before and after photos are only comparable if the whole face is sharp in both.
Depth of field is the range of distances, measured from the camera, in which objects look sharp in the image. Everything inside that range appears in focus; what lies before or beyond it gradually blurs. In a portrait, it determines whether the whole face, from nose to ears, looks sharp or only part of it.
What controls depth of field?
Three factors, which combine:
| Factor | If it increases | Effect on depth of field |
|---|---|---|
| Aperture (f-number) | Higher f-number (more closed) | Greater depth |
| Focal length | Longer lens | Shallower depth |
| Distance to the subject | Camera closer | Shallower depth |
On phones, lenses have a fixed aperture and a small sensor, which generally gives a large depth of field. That is why the blurred-background effect of "portrait mode" is produced by software, which estimates what is face and what is background and blurs the latter.
Why does it matter in clinical photography?
The goal of a clinical photo is to record the face faithfully, for planning and comparison. For that, the areas to be analyzed must sit in the same zone of sharpness: glabella, eyes, nose, mouth, chin and jawline.
When the depth is shallow, the nose may be sharp while the sides of the face are not. Reading texture, volume and contour becomes less reliable, and the before and after stop being comparable if each photo was taken with a different focus.
How to standardize without expensive equipment?
The biggest gain comes from repetition. Keep the same distance between the device and the patient, at eye level, with the patient in a neutral position, both at rest and smiling. A simple tripod or a mark on the floor solves most of the variation.
Avoid getting too close: besides reducing depth of field, bringing the camera too near distorts proportions, making the nose look larger relative to the ears. That effect interferes with facial anthropometry measurements and with reading symmetry.
And in Harmy?
The app asks for a frontal photo, and its quality influences face detection and the marking points. A sharp, well-lit, front-facing photo helps the app locate the areas accurately.
Also called: DOF (depth of field), focus depth (informal use).
Frequently asked questions
Does the phone's portrait mode get in the way of a clinical photo?
It can. It blurs the background by software and sometimes the edges of the face and hair, which makes contours harder to read. For a clinical photo, prefer the regular photo mode with good light.