How camera lens reflections work

Why a lens sends light back

A flat mirror obeys a simple rule: light arrives at an angle and leaves at the mirror angle on the other side. Shine a torch at a mirror from the side and the reflection goes off in the opposite direction, away from you. To see it, you have to stand where the light is going.

A camera is built differently. Its lens focuses incoming light onto a small sensor, and the sensor and the surfaces inside the lens assembly reflect a portion of that light back through the same optics. The lens then does to the outgoing light what it did to the incoming light, but in reverse: it collimates the reflection back along roughly the path it arrived on. The technical name for sending light back towards its source is retroreflection, and it is the same principle that makes cat's-eyes on a road, high-visibility jackets and bicycle reflectors light up in headlights while everything else stays dark.

The practical consequence is the important part. The returned light is concentrated in a narrow cone pointing back at the light source. Your eye has to be inside that cone to see it. If your eye is far from the torch, the light goes back to the torch and not to you.

Why the viewing angle matters so much

This is the single thing people get wrong. Holding a torch at arm's length and scanning a room from a distance does not work. The technique requires three things to be nearly collinear: the light, your eye, and the object.

The same geometry explains why a lens can be completely invisible from one position and glaringly obvious after you shift 10 cm. It also explains why the technique needs the object to be reasonably close. The further away you stand, the tighter the alignment has to be in angular terms and the dimmer the return, because the returned light spreads and the object subtends a smaller angle.

Using a phone helps for a mundane reason: the camera and the torch on the back of a phone sit a centimetre or two apart, which is a much tighter light-to-viewpoint separation than your eye and a hand-held torch. It also lets you hold the view steady and look at the screen rather than into a bright light.

Why ordinary objects reflect too

Plenty of things return a bright point when you shine a light at them, and you will find several in any room. This is the main practical difficulty with the method, not an edge case.

• Polished screw heads. A curved metal screw head reflects across a wide range of angles, so it stays bright as you move — the opposite of a lens's narrow behaviour.
• Glass and glossy plastic. Clock faces, picture glass, TV bezels, plastic domes on smoke detectors, varnished wood, gloss paint.
• LEDs. An LED has a small clear lens over it, which retroreflects in much the same way a camera lens does. Standby indicators are a common false positive.
• Infrared and motion sensors. The lensed windows on presence detectors, thermostats and remote-control receivers behave optically like small lenses.
• Metal trim, mirrors and mirrored surfaces, which throw back large bright areas rather than points.
• Water droplets and dust on a surface, briefly.

Practical technique

1. Dim the room. Not pitch dark — you need to see what you are pointing at — but low enough that a small point of light stands out against the object.
2. Choose your target objects first, using sightlines. Anything with a clear view of the bed, shower or changing area. There is no value in glinting the underside of a wardrobe.
3. Bring the light close to your viewpoint. Phone torch and phone camera, or a torch held against your temple.
4. Stand one to two metres from the object. Closer for small objects.
5. Sweep slowly across the object's surface, then move your head or the whole phone laterally by 5 to 15 cm while keeping the light aimed at the same spot. You are testing how the highlight behaves as the angle changes, not just whether it exists.
6. Repeat from a different height. A lens angled downward at the bed will return best from a position roughly on its own axis, which may not be standing eye level.
7. For anything that flickers on and off with angle, walk up and look at the object directly with your eyes at close range. That final look is what actually settles it.

Expect false positives every time. A room where nothing glints usually means you have not been close enough or have not varied the angle enough.

What Findy can help with

• The lens-glint finder puts the camera view and the torch together with a display tuned to make a small bright point easier to notice than it is on a normal camera preview — particularly against a busy or patterned background.
• The automatic lens scan watches the camera feed for the small, sharply bounded bright points characteristic of a glint, so you can concentrate on moving the phone steadily rather than staring at the screen.
• Steadiness and framing. Looking at a screen rather than into a torch beam makes it much easier to hold the angle and to do the lateral movement deliberately.

Both camera tools require the subscription, $39.99 per year in the US.

What Findy cannot determine

• Whether a reflective point is a lens. Findy reports that something reflected; it cannot classify the object. The label is Reflective point detected, never a confirmation.
• It cannot tell a camera lens from a screw head, an LED, a sensor window or a piece of glass. The optical signature overlaps.
• It cannot see a lens that is covered, painted over, heavily recessed, behind a smoked or tinted panel, or dirty enough not to return light.
• It cannot find a lens you never point the camera at. This is an object-by-object inspection method, not a room sweep.
• It cannot compensate for a bad angle. If your alignment is off, there is nothing in software that recovers a reflection that never came back.
• It cannot see a powered-off device any better or worse than you can — this method does not depend on power at all, which is its one advantage over network and radio scanning.

Related guides

• Can an iPhone detect infrared? — the other optical method, and why it depends on your specific hardware.
• How to check a hotel room — where to point the light, in what order.
• How to check a holiday rental.
• The camera inspection tools in detail.