- TECHNIQUES: DYNAMIC RANGE -
As we look around a scene, the irises within our eyes can adjust to changing conditions as we focus on regions of varying brightness—both extending the dynamic range where we can discern detail, and improving the local contrast. This is apparent when we stand near a window in a dark room on a sunny day and see not only detail which is indoors and around the window (such as the frame or the pattern on the curtains), but also that which is outside and under the intense lighting (such as the blades in the grass in the yard or the clouds in the sky).
Cameras, on the other hand, cannot always capture such scenes where the brightness varies drastically—at least not with the same contrast as we see it. Traditional landscape photography has practiced a technique to overcome this limitation by using a filter which lets in more light in the darker regions, and less light in the brighter regions, resulting in an evenly exposed image throughout.
This works remarkably well, however it is limited to photos with a simple distribution of light intensity, since a filter has to exist which approximates the light distribution. This usually limits the photographer to photos consisting of a bright sky, which gradually transitions into a darker foreground, such as the filter shown below.
Other scenes, such as those which contain alternating electrically-lit and moonlit objects, contain far more complex lighting geometries than can be captured using traditional photographic techniques. To increase the dynamic range captured in a photo, while still retaining the local contrast, one can expose the photo several times: once for each region where the light intensity changes beyond the capabilities of their equipment. This is adapted from a similar technique used in astrophotography. By exposing the photo several times for each intensity region, one then has the ability to combine these images for any arbitrary lighting geometry—thus diversifying the types of scenes one can reproduce in photographic print.
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