Friday, 22 July 2016

Transparent figures in photos

HDR ghostAnyone taking a picture and seeing a transparent figure in the resulting photo might feel that a ghost was worth considering as an explanation. But it may, in fact, have a natural explanation (apart from a long exposure). Indeed, such photos may become increasingly common thanks to a feature found in many modern digital cameras and phones. It is called HDR or high dynamic range imaging.

Digital photography has a relatively low dynamic range compared with the human eye. This means that details, visible to the naked eye, can be lost in shadows or brightly lit parts of a digital photo. HDR fixes this problem by taking multiple photos (usually 3), at different exposure settings, when the shutter button is pressed once. These photos are them combined to produce a single picture revealing details in dark and light areas of the picture. Depending on the setting, you can either save all three photos, and combine them later in photo editing software, or it can be done automatically in the camera at the time of exposure.

When HDR pictures are produced in the camera, it is possible that the photographer may forget that they were taken that way. Unfortunately, the fact that HDR mode was used may not always be visible in the EXIF data. It should be obvious to the photographer at the time that HDR was used but that fact might be forgotten later. One clue might be if there is some motion blur when the shutter speed appears fast enough to stop it. A transparent figure may appear in HDR photo because someone moved during the extended time it took for the three shots to be taken.

The photo (above right) shows a HDR shot of ASSAP's own Seriously Strange magazine. In front of it there is blurry red object that appears partially transparent. It is a red cord swinging through the picture while the 3 shots were being taken.

The 'HDR ghost problem' may well solve itself over time. It is likely that HDR software in future will automatically delete objects that don't show up in all three shots. The problem may eventually become restricted to certain older camera and phone models.

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