Monday, August 20, 2007
Essential Processing Techniques
Tools all photographers should know for adjusting exposure, color and sharpening
By Dave Willis
Understanding exposure and tonality is crucial to getting the best images that you can. The light you record at capture is limited by the dynamic range of the sensor and dictates the details and tonality throughout the shadows, highlights and midtones of the image. While you can't re-create what wasn't captured, you can use software to adjust the exposure of your image to enhance what's captured.
Because digital camera sensors record a limited dynamic range compared to that of the human eye, we make trade-offs. When exposing for details in shadows, for instance, the highlights may blow out. Conversely, when exposing for the details in highlights, the shadows can crush. It's important to be aware of this when you're taking the shot and then use software to make the most of your exposures.
Photoshop's exposure controls are among the very best, and they got better in version CS3. In CS2, there's an exposure slider adjustment, Image > Adjustments > Exposure, and CS3 has extended its capabilities. There are three sliders: Exposure for highlights, Offset for darkening of shadows and midtones, and Gamma to set luminance levels. Droppers can also sample and establish black, white and gray areas in the image with one-click adjustment.
The Curves box, Image > Adjustment > Curves, has been a consistent tool in the Photoshop lineage. Curves displays tones as a graphic line with plotted anchor points, stretching from dark to light along an input and output axis. Changing the plot points and manipulating the intersecting tonal line gives complete control over color tone and contrast. There are four color channels for individual or combined tonal shift: Red, Blue, Green and RGB.
CS3 has added functionality to the Curves tool, too. Now there's a histogram included beneath the tonal curve to show you how the curve applies to specific tones in your image. Black and white sliders have been added for easier setting, and CS3 has also added preset templates for standard adjustments, such as high contrast or negative inversion. There's also custom presetting, and display options have been expanded.
High Dynamic Range (HDR) takes bracketed exposures of one scene and merges them into a single image with an optimum exposure. Imagine brights that are just as detailed as the shadows. If you're shooting in RAW, CS2 and CS3 can also do this by processing one image more than once for a combination of different highlight, midtone or shadow areas. For more on this technique, visit our website at www.pcphotomag.com and search for HDR.
Aperture features a multiple-image workflow, so if you need to make similar adjustments to a group of images from a shoot, it's easy. Image adjustments can be saved as customizable presets that can be stamped on any image or group of images. Aperture also provides excellent automatic tonal and exposure adjustments, but the best results come from experimentation with the tools. All of your edits in Aperture are nondestructive, meaning they aren't actually applied to the original image, but instead are saved as a kind of metadata.
Aperture's Exposure tool corrects exposure, saturation, brightness and contrast by using sliders or numeric values. It's a simple solution with complex possibilities. Once you like the adjustment settings you've made, they can be recorded as presets to use again and again. Working with a properly calibrated monitor, these sliders make easy work of complicated adjustments.
The Levels tool of Aperture uses a series of pendulum-like sliders to adjust the tonal values of shadows, midtones and highlights. Values can also be set numerically, and for tighter control, Quarter-Tone Levels can be selected for tonal values between midtones and the highlights or shadows.
Adobe Photoshop Lightroom
Even though Lightroom is part of the Photoshop family, the program offers its own unique solutions to image adjustment. The Curves tool of Photoshop has been revamped into the Tone Curve in Lightroom, and though its basic functionality is the same, the Tone Curve offers more of a handheld approach, offering guided tonal change at chaperoned safety levels. Think subtle adjustment for best results with this powerful tool.
The most interesting Lightroom feature is that you can actually adjust tonality by dragging within the image itself. By clicking on the circle in the top left of the Tone Curve box, you can use your cursor to select a point in the image and then drag up or down for brighter or darker tonality. The histogram can be pushed or pulled to compress or expand tone ranges, as well.
For image manipulation on the fly, each click of the Exposure bar in the Quick Develop window of the Library viewing mode will move total exposure up or down by one stop. There's also an Auto Tone button.
If you want more thorough exposure control, there's exposure manipulation in Lightroom's Develop mode. Here you'll find four sliders: an Exposure slider for up to four stops of exposure in either direction, a Recovery slider for reducing extreme highlight tones, a Fill Light for boosting shadow detail and Blacks for heavier black saturation.
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