1 The idea
A ghost is an image of a bright light that has bounced between two glass surfaces inside the lens. That stray light doesn’t focus to a point, so it lands as a blurred picture of the aperture: round with a round iris, polygonal when a bladed iris is stopped down, and cut off by the lens edges when the light nears the frame edge. Each one has its own size and colour, set by the pair of surfaces and their coating. The ghosts here are traced through a Double-Gauss 50 mm lens.
Every bounce happens around the same optical axis, so all ghosts sit on one straight line from the light, through the frame centre, to the other side. Move the light and the far-side ghosts move the opposite way. Drag the light, then switch the coating and the f-stop: bare glass makes each ghost about 10–30× brighter and almost colourless, and at f/5.6 the ghosts shrink and sharpen. The blade count sets the star rays: an even count gives n, an odd count gives 2n.
ghost = C + t · (L − C) · t < 0 → the far side2 Hands-on
Task: place the light so the ghost marked ★ (a small bright one on the far side) sits in the target ring
The ★ ghost sits at t ≈ −0.9: about 0.9 times the light’s distance from the centre, on the other side. Work backwards from the target: light ≈ centre + (target − centre) ÷ (−0.9), then fine-tune, because in a real lens t drifts a little with the light’s position (watch t in the readout).
3 Deep dive
What one flare is made of
What we call “a flare” is really three or four things stacked together. Each forms in a different place in the lens and follows its own rules. Separate them and you know what to build and where it goes.
| Part | Caused by | Looks like |
|---|---|---|
| Ghost | Light reflecting twice between glass surfaces | Iris shapes on a line through the centre |
| Starburst / glint | Diffraction at the iris blade edges | Rays around the light |
| Veiling glare | Light scattered by dust, smudges and the barrel (a smudge can also give a diffusion look, see LA7) | A thin haze, lifted blacks everywhere (see LA4) |
| Glow around the light | Light spreading near the core | Soft bloom around the source |
Why ghosts line up
All the elements in a lens are centred on one axis, so light bouncing between them stays symmetric about it. Every ghost falls on the line from the light through the lens centre: positive t on the light’s side, negative t on the far side, moving against the light.
A lens with S glass surfaces has S(S−1)/2 possible reflection pairs, so a zoom with many elements has more, messier ghosts than a prime. Most are too faint to see; only a few read clearly.
A ghost off the light–centre line looks wrong at once
Shape, colour and size
A ghost is a blurred image of the iris. Wide open, ghosts are round and soft; stopped down, they turn into crisp polygons with one side per blade. Give each one its own size and strength. Colour comes from the coating on that pair of surfaces, and ghost edges usually carry a slight colour fringe; ghosts with none read as CG.
Starburst rays come from the blade edges, two per edge: even blade counts overlap to n rays, odd counts give 2n. Stopping down makes them longer and sharper. A 14-ray star in the plate means a 7-blade iris.
Making a flare believable
A flare exists only while its source is in frame or close to the edge, and it dims when the light is blocked. Flares live on the glass, not in the scene, so a 2D track of the source drives the position and the ghosts run along the line through the centre. How bright the light is changes the flare’s apparent size more than how far away it is.
Keep it restrained: a few big soft ghosts look better than a string of little beads. Judge it in motion, then try half. Add a slight flicker and drive every layer of that light’s flare from the same flicker.
- 1Track the source
- 2Ghosts on the centre line
- 3Strength follows the light (blocked = dimmed)
- 4Merge plus before the shake (LA7)
4 In real work
In Nuke
- Track the light source once and let every layer read its position from that one track. The glow around the light can link to the track directly, but each ghost needs a position expression, centre + t·(track − centre), with its own t (negative for the far side). Link a ghost straight to the track and it just sits on the light.
- Layer several Flare nodes (big soft ghosts, small ghosts, a glow) in their own branch, then Merge plus into the comp in the camera stack: after diffusion and the main glow, before the shake (see LA7).
- Glint for the star rays; set the ray count to match the plate lens’s iris.
- Drive every layer’s Grade gain from one flicker expression, and pull it down when the light is blocked (roto or key of the occluder).
Common mistakes → the fix
- ✗ Placing ghosts freely, off the light–centre line✓ Every ghost = centre + t·(light − centre); move the light and they must slide along that line.
- ✗ All ghosts the same size and colour✓ Vary each one, and add a slight colour fringe at the edges.
- ✗ The flare stays when the light has left frame or is blocked✓ Tie strength to the light’s visibility: no light, no flare.
- ✗ A ray count that doesn’t match the lens✓ Count the plate’s rays. An even-blade iris gives as many rays as blades, an odd-blade iris twice as many, so 14 or 18 rays almost always means 7 or 9 blades.
Before you send the shot
- Every ghost sits on the line through the optical centre, every frame
- Strength follows the light: dims when blocked, gone when out of frame
- Ghost shape and ray count match the lens’s iris
- Judged in motion, then tried at half
5 Check yourself
The light moves up and to the right. Which way do the far-side ghosts move?
🔒 Answer the question above and this card opens
1. The light sits exactly at the frame centre. Where are the ghosts?
2. The plate shows 18 rays around a light. How many iris blades does the lens probably have?
Where the numbers come from: Both rules are published lens physics: ghosts line up through the lens axis, and starburst rays follow even = n, odd = 2n. The positions (t), sizes, colours, brightness and shapes of the 10 ghosts are traced through the same published Double-Gauss 50 mm prescription (Smith, Modern Lens Design; expired US patent 2,673,491) with FlareSim’s open-source optics code (MIT licence), run headless: Fresnel reflections, a single-layer MgF₂ coating, 3 wavelengths, a round iris. The glass Abbe numbers are estimates. On-screen brightness uses a teaching gain; the ratios between ghosts, coatings and f-stops are traced. The glow and star rays around the light are still drawn. Every lens model gives different ghosts. Strictly, the line runs through the lens’s optical centre, which can sit slightly off the frame centre.