LINKS:
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UNDERSTANDING DIGITAL EXPOSURE:
Part 1: Exposure Reduction for Highlight Retention
Part 2: Digital Exposure & Noise
Part 3: When Blocked-up Shadows Aren’t Really
Part 4: Take a Balanced Approach to White Balance
Part 5: Dynamic Range
Part 6: Extending the Tonal Range
Part 7: What’s the Real Difference Between RAW and JPEG?
GENERAL:
1) COST-EFFECTIVE PHOTOGRAPHY
2) CONTROL THE RANGE OF FOCUS
3) IMAGE INTERPOLATION
4) LOSE THE DEAD SHOTS
5) SCANNING 35MM FILM
6) THE RAW vs. JPEG DEBATE…
7) IS PHOTOGRAPHY EASY?
8) MUST EVERYTHING HAVE AN ADOBE SLANT?
9) A CAMERA TO PAINT WITH
10) WHAT'S THIS COMPOSITION THING ABOUT?
Understanding Digital Exposure
6. EXTENDING the TONAL RANGE
By using a tripod and shooting a suitable scene at different shutter speeds it’s possible to extend the tonal range of the final image far beyond the capabilities of the sensor. Unlike graduated neutral density filters* that degrade tonal values by restricting the light before it passes through the lens, this method allows a full exposure of the widest possible range of useful tones.
A graduated filter fitted to a film camera.
In the example above the scene was first exposed for the sky. The intention was to capture as much highlight information as possible. Not all of this detail was necessary for the print but it's best to have a good range of tones to work with. The scene was then shot again for everything else. To retain useful detail towards the very top of the mountain and in the lichen it was necessary to underexpose slightly. (The foreground image had to edited to clone out raindrops on the lens.) The histogram was checked carefully after each exposure.
As shown above the merits of both exposures were brought together in software in one single image. Lichen detail was copied to a separate layer (a free-floating object or adjustment layer) and edited to retain maximum detail for the print.
Below is another example. It was simply impossible to capture the complete range of tones and colours using any other method. Underexposure held the subtle highlight petal tones that would otherwise have been lost. Exposing for the flowers only would have blown much of the out-of-focus background detail. The tonal strengths of each exposure were brought together in one image (bottom-right).
The final image after further editing for digital printing:
The image above is a manual combination of three separate shots, each exposed differently to capture a range of tones visible to the naked eye in the evening light. Care was taken to capture detail in those areas where the dappled light was brightest.
The handheld image below left was shot at a slightly reduced exposure to hold tonal information in the sea and clouds. The RAW data was processed and converted three times to extend the central tonal range and so produce a more realistic result.
The image of a waterfall was created from two slightly different handheld shots. Reduced exposure meant it was possible to edit the very bright tones in the water that had caught the morning sunlight. The brightest waterfall detail was edited separately and positioned in place as a new floating object. It was then carefully shaped by the erasure tool and made to look more realistic by a slight adjustment to its opacity. This allowed the featureless bright areas to show through.
A natural blue cast was removed from the masked off white water lower section by targeting cyan and blue data only. Contrast and brightness adjustments strengthened the tonal range in some shadow areas to the left. Colour was selectively enhanced throughout the image.
Dedicated Software Solutions
The examples below were created from unedited JPG originals using special software designed to merge two or more differently exposed photos into a single image "with increased dynamic range". This means the complete range of tones available in two or more images is used to create a single image.
Extending the range of tones in this way can result in images that look tonally unrealistic, so it's best to experiment if more accurate representations are what you have in mind.
* Neutral density graduated filters need to be used with care. Often they are not suitable for scenes that include anything that runs up through the frame – like trees, poles, steeples, signs, masts, towers, and so on. Unfortunately these will darken gradually too. Correcting affected texture in software is likely to be difficult if not impossible.
But bringing together two separately exposed images as explained above, means the tones can be preserved throughout the image if required. To achieve a natural result in a single image it’s possible to paint in or out relevant areas of the layer, or use the erasing tool, to within a single pixel if necessary. Subtle cloning can tidy up any small problems that remain.
Yet another advantage with creating graduated effects in software is the possibility of adjusting the opacity of the darker image. This is the digital equivalent of choosing different grades of graduated filters in the field, but is obviously more convenient. Simply graduating the image as shot may not produce the most pleasing result. In the above example the graduated layer was set to 40% opacity. The entire effect took just several minutes working in a comfortable environment.
The fact that many professional photographers prefer to use filters may have more to do with financial considerations – time is money, so getting a good result on location makes sense. It’s something for the amateur to consider before accepting filter advice from professionals. It's worth remembering too that some pros may not have the time (or inclination) to learn good software techniques.
