Double contours or reflections in the lens from poor cementing or internal reflections — visible against light or bright sources. Quality defect in old or damaged glass.
Anyone shooting towards the sun with older lenses or damaged glass knows the phenomenon: suddenly, double contours appear, ghost images of the subject — offset, semi-transparent, disruptive. This is caused by internal reflections between the lens elements. Light not only hits the sensor but bounces back and forth between the cemented lens surfaces before reaching the chip. The older the coating or the more damaged the surface texture, the stronger the effect.
On set, you notice it immediately: if you point the camera at a strong light source — sunlight, spotlights, windows — these transparent shadows of the subject appear. They are usually offset from the original contour, appearing like a slight double exposure effect. In extreme backlight, the entire image can become milky. The reason lies in the lens cementing itself. The individual glass elements are joined together with special adhesives — and this cementing ages. The optical surfaces lose their precision, the anti-reflective coating crumbles. This is a classic problem, especially with lenses from the 1970s or 80s.
Practically speaking: Old zoom lenses are more prone to this than modern prime lenses because they have more lens elements — more surfaces, more chances for reflection. A damaged lens surface (scratches, wear) exacerbates it. You can't simply cut out the ghost image — it's in the image itself. The best solution is prevention: avoid backlight or change the angle so that the strong light source doesn't shine directly through the lens. A matte box with a flag, diffusion, or lens hood often helps. Some DoPs consciously accept the effect — vintage aesthetic — but this should be a creative decision, not an accident.
When buying a lens, this is a purchasing argument: damaged cementing is expensive to repair and permanently degrades image quality. Modern lenses with better coatings have this problem significantly less. If you regularly shoot into the light, it's worth testing before production — just hold it up to a bright lamp and look. Blurry ghosts are normal, but double contours are a deal-breaker for professional work.