Adobe Photoshop CS5 for Photographers - Martin Evening [88]
Figure 3.28 To manually set the white balance, select the white balance tool (circled) to locate what should be a light gray neutral area and click to update the white balance.
Color temperature
Color temperature is a term that links the appearance of a black body object to its appearance at specific temperatures, measured in kelvins. Think of a piece of metal being heated in a furnace. At first it will glow red but as it gets hotter, it emits a red, then yellow and then a white glow. Indoor tungsten lighting has a low color temperature (a more orange color), while sunlight has a higher color temperature and emits a bluer light.
There are also calibration charts such as the X-Rite ColorChecker chart, which can be used in carrying out a custom calibration, although the light gray patch on this chart has been regarded as being a little on the warm side. For this reason, you may like to consider using a WhiBal™ card ( Figure 3.27 ). These cards have been specially designed for obtaining accurate white point readings under varying lighting conditions.
Figure 3.27 WhiBal™ cards come in different sizes and are available from RawWorkflow.com .
The tone adjustment controls
The tone adjustment controls allow you to make further adjustments to the highlight and shadow clipping points as well as the overall tone balance and brightness. You can adjust these sliders manually, plus you can click on the Auto button to auto-set the Exposure, Recovery, Blacks, Brightness and Contrast settings.
How Camera Raw calculates
Camera sensors have a linear response to light, and unprocessed raw files therefore exist in a ‘linear gamma space’. Human vision on the other hand interprets light in a non-linear fashion, so one of the main things a raw conversion has to do is to apply a gamma correction to the original image data to make the correctly exposed, raw image look the way our eyes would expect such a scene to look (see Digital exposure on pages 186–187). The preview image you see in the Camera Raw dialog presents a gamma corrected preview of the raw data, while the adjustments you apply in Camera Raw are in fact being applied directly to the raw linear data. The reason I mention this is to illustrate one aspect of the subtle but important differences between the tonal edits that can be made in Camera Raw to raw files and those that are applied in Photoshop where the images have already been ‘gamma corrected’. Note that in the case of non-raw files, Camera Raw has to temporarily convert the image to a linear RGB space to carry out its calculations.
Exposure
The Exposure slider is used to set the overall brightness, so that the photograph looks ‘correctly exposed’. You can use the Exposure slider to visually assess how brightly exposed you want the picture to be and use this as your primary tool for adjusting the image brightness. If you hold down the key as you drag the Exposure slider this shows you a threshold preview which indicates where there might be highlight clipping, or you can rely on the highlight clipping indicator shown in Figure 3.30 to tell you which highlights are about to be clipped.
Figure 3.30 If you click on the clipping indicators in the Histogram panel you will see a colored overlay in the preview image that indicates any shadow and highlight clipping.
Recovery
You don't have to worry too much about setting the Exposure too bright or clipping the highlights, because the Recovery slider can be used to restore some of the detail which at first may appear lost. The Recovery slider cleverly utilizes the highlight detail in whichever channels contain the best recorded highlight detail and uses this to boost the detail in the weakest highlight channel. There are limits as to how far you can push a Recovery adjustment, but you may be able to recover as much as a stop or more of overexposure. Camera Raw can sometimes use extra tricks such as ignoring digital gain values used to create higher ISO captures. If you hold down the key as you drag the Recovery slider