Or, How to Make Your Camera Work More Like Your Eyes
Anyone who has ever tried to snap a few photos with a modern camera has probably felt the frustration that the camera often fails to get the exposure quite right, with things that you can see clearly either washed out and over-exposed, or invisible in the shadows. The intrepid among you may have even gone so far as to turn the switch to manual, only to discover that it is really hard to do too much better than the automatic system. You end up with these types of exposure-bracketed images:
The sky detail is only visible in the lower exposure (faster shutter speed) and the building detail is only visible in the higher exposure. And yet when you look directly at the scene, everything is clearly visible to the naked eye. The reason for this is that through an amazing combination of the iris' aperture dilation, and the logarithmic response of the photo-receptive neurons in retina, the human eye automatically adapts to an incredibly broad range of illumination intensity as it scans across broad scenes. A camera, on the other hand, must use the same exposure aperture and sensitivity settings across the entire image, and the photo-detectors have a linear response curve.
As a result, a camera's dynamic range (the number of discrete brightness gradations it can detect) is much more limited, and the same range is applied across the entire field-of-view. This means that in scenes with wide illumination variations, typical photographs fail to capture details on either the bright side, or the dark side.
But there is a growing field of photography called "High Dynamic-Range Imaging" that combines multiple photos of varying exposure to artificially expand the dynamic range of a final composite image. The technique has demonstrated amazing results. Here is the final photo created using the three exposures above. (examples from this site)
This photo shows a great solution to the canonical problem of imaging a bright outdoors from inside a much darker room. Here is an example applied to portraiture:
There is definitely an art to assembling the images, because if the technique is applied to aggressively, the resulting images look a little surreal. Here are some more examples from a great web site that are on the edge, but still very interesting.
Modern HDR Photography
Go forth and process.