Monday, September 6, 2010

HDR – part1

The problem with today’s digital sensors
Digital photography has gone a long way in the last decade. Today’s digital cameras offer amazing capabilities and exceptional image quality that it was difficult to even imagine a few years ago. Digital has change everything; darkrooms have become “Lightrooms”, film has become 64 GB flash disks (take as much shots as you want; it’s free!), new and intelligent matrix metering systems, extraordinary ISO capabilities (at a rotate of a wheel, not by changing films all the time), new lenses with CPU, 10 fps, digital printing (a lap at home), etc, the list is endless. However, there is one major problem; the restricted dynamic range (or latitude).   The problem is due to the linear nature of sensors, meaning that digital has no tolerance to metering mistakes; highlights are blown very easily. A modern DSLR has about 10 – 11 stops of dynamic range at the base ISO (the range is being decreasing as ISO values are becoming higher), meaning that the sensor is able to record 10-11 stops of image data from dark to bright values:
 e.g. in the following image, we have pixels that are either completely dark or completely white (no detail). The histogram shows the distribution of these tones:
If we try to photograph a scene with a greatest dynamic range (contrast) than our camera’s dynamic range, the results are rather disappointing:

(a)          If we meter for the highlights (sky), darker subjects (tree, ground) get underexposed.
(b)          If we meter for the darker subjects, brighter subjects get blow out.
(C)          If we take a an overall reading, we end up with compromise results.

“Traditional” Solutions
1.            Use light (flash, reflectors, softboxes) to illuminate darker subjects.
2.            Use filters (ND graduate, polarizer) to darken the highlights.
3.            Use Fill light & Recovery (camera raw) and Selective Adjustments in Photoshop, to darken the highlights and brighten the dark tones.
4.            Use Dodging & Burning in Photoshop, to darken the highlights and brighten the dark tones.
5.            Use Exposure Blending in Photoshop (blend different exposures in separate layers and selectively erase certain areas).
6.            Contrast Masking in Photoshop.
7.            New Technologies, like Nikon D-lightening.
8.            You can photograph during the “blue hour” of the day, where the sky and the ground have the same amount of brightness.
! Use the above methods with caution, as they may result in contrast loss!

Coming next: the HDR methodology.

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