Inphotography,backscatter (also callednear-camera reflection[1]) is anoptical phenomenon resulting in typically circularartifacts on an image, due to the camera'sflash being reflected fromunfocused motes ofdust, waterdroplets, or otherparticles in theair or water. It is especially common with moderncompact and ultra-compact digital cameras.[2][3]
Caused by thebackscatter of light by unfocused particles, these artifacts are also sometimes calledorbs, referring to a commonparanormal claim. Some appear with trails, suggestingmotion.[4]
Backscatter commonly occurs in low-light scenes when the camera'sflash is used. Cases includenighttime andunderwater photography, when a bright light source and reflective unfocused particles are near the camera.[1] Light appears much brighter very near the source due to theinverse-square law, which says light intensity is inversely proportional to the square of the distance from the source.[5]
The artifact can result from thebackscatter orretroreflection of the light fromairborne solid particles, such asdust orpollen, or liquid droplets, especially rain or mist. They can also be caused by foreign material within the camera lens.[2][4] The image artifacts usually appear as either white or semi-transparent circles, though may also occur with whole or partial color spectra,purple fringing or otherchromatic aberration. With rain droplets, an image may capture light passing through the droplet creating a smallrainbow effect.[6]
Fujifilm describes the artifacts as a common photographic problem:
There is always a certain amount of dust floating around in the air. You may have noticed this at the movies when you look up at the light coming from the movie projector and notice the bright sparks floating around in the beam. In the same way, there are always dust particles floating around nearby when you take pictures with your camera. When you use the flash, the light from the flash reflects off the dust particles and is sometimes captured in your shot. Of course, dust particles very close to the camera are blurred since they are not in focus, but because they reflect the light more strongly than the more distant main subject of the shot, that reflected light can sometimes be captured by the camera and recorded on the resulting image as round white spots. So these dots are the blurred images of dust particles.[2]
In underwater scenes, particles such as sand orplanktonicmarine life near the lens, invisible to the diver, reflect light from the flash causing the orb artifact in the image. A strobe flash, which distances the flash from the lens, eliminates the artifacts.[7] The effect is also seen oninfrared video cameras, where superbright infraredLEDs illuminatemicroscopic particles very close to the lens. The artifacts are especially common with compact or ultra-compact cameras, where the short distance between the lens and the built-in flash decreasesthe angle of lightreflection toward the lens,directly illuminating the aspect of the particles facing the lens and increasing the camera's ability to capture the light reflected from normally subvisible particles.[2]
Someghost hunters have claimed that orb shaped visual artifacts appearing in photographs are spirits of the dead.[8][9] Such perceptions have been interpreted byMichael Shermer as examples ofagenticity.[10] Prominent paranormal investigators such asJoe Nickell have agreed with skeptic-debunkers' assessments that orbs result from natural phenomena like insects, dust, pollen, or water droplets.[11][12]
An additional problem called backscatter occurs when flash light striking these suspended particles reflects back to the camera lens and records on the film as fuzzy white spots. Of course, backscatter can also be reduced by getting the camera as close to your subjects as possible, because the shorter that distance, the fewer the number of floating particles in front of the lens.
Media related toPhotographical orbs at Wikimedia Commons
