Monday, March 3, 2008
Introduction To XDR
Extending the dynamic range in your images is possible with sound technique and a little software magic
Start With A Good Exposure
A primary concern during exposure is ensuring that exposure provides adequate information in deep shadow detail. Digital cameras respond to light differently than human eyes. Digital cameras count photons and assign a tonal value that's directly proportional to the number measured. Human eyes are more sensitive to differences in brightness at low levels than at high levels. Relating the numbers in a digital image to the perceived brightness they represent is called gamma encoding. Digital cameras respond linearly (a gamma of 1.0 or a straight line on an input-output graph), while human eyes don't (a gamma between 2 and 3, depending on viewing conditions or a steep curve). Darkening moves more data into shadows while lightening spreads fewer values over a greater range, which may exaggerate noise and increase tendencies toward posterization.
One big advantage to shooting digitally is the ability to view a histogram in the LCD screen on the back of your camera body. A histogram is a graph of the tonal distribution in an image, displaying shadows on the left and highlights on the right. Once you view the histogram, you can determine if you need to make additional exposures at alternate settings to get better data. To use this feature, simply program your camera to display a histogram immediately after exposure. Having this immediate feedback will result in a significantly higher success rate.
When evaluating a histogram, the primary concern is clipping, or loss of data due to underexposure or overexposure. When a histogram “hits the wall” to the left, the image is underexposed. If the histogram “hits the wall” to the right, the image is overexposed. Either scenario indicates that you should change exposure settings to get a more balanced exposure for your image.
The histogram displayed in the LCD represents the information of an image in a converted JPEG state, even if you're shooting in RAW. Because RAW is so flexible, you won't truly know what the histogram of a RAW file will look like until it has been processed. Many digital cameras will allow you to set JPEG contrast to a low setting, which reduces the likelihood of clipping in JPEGs and provides a better RAW preview.
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