What Is a Lens Flare?

What is actually happening

An image is formed by light that travels the intended path through the lens. Some light does not. It reflects off the surfaces of the glass elements, bounces off the inside of the barrel, and eventually reaches the sensor somewhere it does not belong. That misplaced light is a lens flare, and it takes a few distinct forms:

• Veiling glare. No visible shape, just an overall milky lift. Blacks go grey, contrast collapses, the image looks fogged. This is the version that ruins shots without anyone noticing why.
• Ghosting. A chain of coloured polygons or discs marching across the frame, mirrored around the center from the light source. The polygon count matches the aperture blades.
• Streaks and starbursts. Radiating spikes from a point source, again produced by the aperture blades when stopped down.
• Anamorphic streak. A single horizontal line, classically blue, spreading out from the source. Different mechanism: the cylindrical element scatters along one axis.

Every one of these is an artifact. They exist because lenses are imperfect, and they got adopted as a language precisely because they signal a physical camera looking at a real light.

Why it gets used on purpose

Flare tells the audience there is a camera in the scene. That is worth something in documentary and found-footage work, where you want the frame to feel witnessed rather than composed. It also does practical jobs: a streak across a silhouette separates the figure from the background, and a veil of glare in a bright exterior sells heat better than colour grading does.

The counter-case is just as real. Flare eats contrast, so it destroys texture in skin and fabric. It also does not cut well: a flare that appears in one angle and not its reverse draws attention to the edit. And in dialogue, anything moving across a face pulls the eye away from the performance.

Prompting one

Two things decide whether you get flare that looks placed rather than pasted: naming the light source and naming its position relative to the frame.

• ✅ low sun just outside the top-left of frame, horizontal blue anamorphic lens flare across the shot
• ✅ single practical bulb behind the subject, warm ghosting toward the lower right
• ⚠️ lens flare alone usually gets you a generic overlay floating in the middle of the image
• ⚠️ cinematic lighting gets you flare whether you wanted it or not

For video, add a motion cue. A flare that stays nailed to the same pixel while the camera moves is the clearest sign the effect is composited rather than optical. Asking for the streak to sweep as the camera pans gives the model a reason to move it.

What goes wrong in generated frames

The recurring problem is a flare with no cause. Check that the bright source implied by the streak actually exists in the frame or plausibly sits just outside it, and that the streak points away from that source rather than at a random angle. Second, look at what the flare covers: real flare sits over the whole image as added light, so it should never have a hard edge or vanish behind a foreground object.

In video the failure is temporal. Flares often flicker, jump between frames, or appear and disappear at a cut in the middle of a clip. Shorter generations and a slower camera move both reduce it. If a shot is otherwise good, adding the flare in post is usually faster than rerolling for one that behaves.