Adobe Photoshop CS4 - Manual (Page 186)

179 USING PHOTOSHOP CS4
Color and tonal adjustments
Last updated 1/10/2010
2 In the Adjustments panel, set any of the following options:
Exposure Adjusts the highlight end of the tonal scale with minimal effect in the extreme shadows.
Offset Darkens the shadows and midtones with minimal effect on the highlights.
Gamma Adjusts the image gamma, using a simple power function. Negative values are mirrored around zero (that is,
they remain negative but still get adjusted as if they are positive).
The eyedroppers adjust the luminance values of images (unlike the Levels eyedroppers that affect all color channels).
• The Set Black Point eyedropper sets the Offset, shifting the pixel you click to zero.
• The Set White Point eyedropper sets the Exposure, shifting the point you click to white (1.0 for HDR images).
• The Midtone eyedropper sets the Exposure, making the value you click middle gray.
More Help topics
“ About high dynamic range images ” on page 71
Targeting images for press
Setting highlight and shadow target values
Assigning (targeting) highlight and shadow values of an image is necessary because most output devices (usually
printing presses) cannot print detail in the blackest shadow values (near level 0) or the whitest highlight values (near
level 255). Specifying the minimum shadow level and maximum highlight level helps to bring the important shadow
and highlight details within the gamut of the output device.
If you are printing an image on a desktop printer and your system is color-managed, don’t set target values. The
Photoshop color management system automatically makes adjustments to the image you see on the screen so that it
prints properly on your profiled desktop printer.
Using Levels to preserve highlight and shadow details for printing
The Output Levels sliders let you set the shadow and highlight levels to compress the image into a range less than 0 to
255. Use this adjustment to preserve the shadow and highlight details when an image is being printed on a press whose
characteristics you know. For example, suppose there are important image details in the highlights with a value of 245,
and the printing press that you’re using won’t hold a dot smaller than 5%. You can pull the highlight slider to level 242
(which is a 5% dot on the press) to shift the highlight detail from 245 to 242. Now, the highlight detail can safely print
on that press.
Generally, it is not a good idea to use the Output Levels sliders to target images with specular highlights. Your specular
highlight will look gray rather than blow out to pure white. Use the highlight eyedropper for images with specular
highlights.