2011

NOTE Many of the Photo Lab tool's fields are animatable attributes—see Setting
Keys Manually
on page 676 and Validating and Applying the Expression String on
page 729.
Setting Exposure, Contrast, Pivot Point, and Lift of an
Image
■ Exposure Exposure can be expressed in terms of F-stops, printer lights,
or gain. As with cameras, increasing the exposure by one F-stop results in
an image with the luminance value that is double that of the original.
Printer light units work very much like F-stops, but it usually takes six
printer light units (default setting) to get the same effect that you get if
you set exposure by one F-stop. If you are trying to simulate a calibrated
printing device for which you have accurate measurements, Photo Lab
allows you to specify how many printer lights there are in one F-stop.
Increasing the exposure by one F-stop also increases the gain by a factor
of two.
■ Contrast and Pivot Gamma correction is applied about a pivot value.
In other words, pixel values equal to the pivot value are left unchanged.
This is useful when you want to use gamma correction to change the
contrast of an image, but do not want to affect a particular luminance
level. The amount of gamma correction can be expressed in terms of
contrast or in terms of the usual gamma exponent. Increasing the gamma
exponent decreases contrast.
■ Lift The lift is simply a uniform offset added to all the pixel values
(different offsets for different color components). Lift is usually applied as
the last step of the process and can be used to control the overall brightness
of the image.
NOTE This tool simulates the physical workings of camera exposures and printing
devices as long as the media is encoded in a linear color space. All channels are
linked by default.
Use the following procedures to set values for exposure, contrast, pivot point
and lift. Note that menus are available for selecting exposure and contrast
units.
Setting Exposure, Contrast, Pivot Point, and Lift of an Image | 647