Photoshop Filter and Tool Tutorials
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Using Photoshop Curves to Adjust Color & Tone
This Photoshop tutorial is designed to instruct on the use of Photoshop's Curves dialog box palette window for adjusting color balance, tone, and the setting of a correct white point. The Curves adjustments window is one of the most powerful, and misunderstood tools in the Photoshop arsenal. Note: It is critical that you have a properly calibrated monitor to perform these subtle alterations of your photo images.
Unlike the "Levels," "Brightness/Contrast," and 'Hue/Saturation' which can effectively destroy or compress pixel and highlight/shadow information, Curves is an elegant way to fine tune the tonal quality of your photos without damage to the subtle shadow and highlight pixel information. Although there has been improvements to the Brightness/Contrast feature, Curves is much more powerful and intuitive. Once you have mastered Curves, you will never again use Brightness/Contrast or Levels.
CS5 Technical Note: Several improvements have been made to the "Curves" dialog box window in CS5, including the addition of a histogram graph that underlies the Curves window, and the addition of "Channel Overlays" which show any adjustments you have made to individual channels when you are in CMYK or RGB mode.
Using Curves Diagonal Box Graph Slider
You can use the Curves diagonal slider line to adjust color globally in RGB, CMYK, or Grayscale (dependent on the Image Mode you are using), or you can make adjustments to individual Channels by selecting them from the pulldown menu at the top of the Curves dialog box.
In the default setting, the upper-right corner of the graph represents highlights, and the lower-left corner of the graph represents shadows. By randomly selecting any point along the diagonal line you will create anchor-points on the line. You can make adjustments to the values of the pixels within that tonal range on the graph by dragging your newly-created anchor-points upward or downward. By dragging the line up in the default setting you will add value to those pixels, and by dragging the line downward you will decrease the pixel value within that range.
Technical Notes: While you are in the Curves dialog box you can scroll over the photograph image and the cursor will change into an Eyedropper Tool. By selecting any part of the photograph you will visually see where that tonal area is located on the diagonal graph line. Always set the Eyedropper Tool sample-size to 5 x 5 pixels, especially important with photos that contain a high amount of noise.
By dragging the anchor-point upward while in the "single channel" mode, it will give the impression of "adding" intensity to the selected color, while the remaining colors that are not adjusted. By dragging the anchor-point downward while in the "single channel" mode you will lessen the intensity of the selected color, giving the impression of increasing the intensity of the remaining colors.
Technical Note: If at any time you are unhappy with the results of your corrections you can hold the Option key which will change the Curves dialog box 'Cancel' button into a 'Reset' button.
Using Curves to Increase Contrast
You can use the Curves diagonal slider line to adjust contrast globally in RGB, CMYK, or Grayscale by selecting the default anchor-points at the furthest end of the diagonal graph line, and moving them upward or downward to increase, or decrease contrast.
By dragging the highlight anchor-point (upper right corner) to the left while in the "global" RGB or CMYK mode, you can increase the white point globally (example 'A,' above). Conversely, by dragging the shadow anchor-point (lower left corner) to the right while in the "global" RGB or CMYK mode, you can increase the black point globally (example 'B,' above). Reversing these procedures will decrease contrast of the photograph.
If you are happy with the highlight target white point, and shadow target black point, but you still want to increase or decrease contrast, you can add an anchor-point to the center of the diagonal line and two additional anchor-points to the in-between points (example 'C,' above), and drag them in opposite directions.
Using Curves Auto Color Correction Options
You can always let Photoshop use its auto correction algorithm to do the thinking for you, and sometimes this can yield good results. By selecting the "Options" button to the right of the Curves dialog box you will bring up the Auto Color Correction Options dialog box.
By selecting the Shadows, Midtones, and Highlights box you will bring up the Photoshop Color Picker palette. By sliding the Circle Cursor in the "Select Target Highlight Color" box you can manually play with the target color and brightness of the White Point. See this link for additional info on setting a photo's White Point with Curves.
When all of your corrections have been made to your duplicate layer (described in the fifth paragraph (Technical Notes), assess the results by toggling the layer on and off so that you can compare your altered version to the original version below. I have found that the more corrections you make, and the further you stray from the original photograph, you can easily overdo the corrections. If this is the cast it is an easy fix as you can dial down the opacity of the top altered layer until you have a "natural" looking result.
Technical Note: By selecting a specific area of the photograph with the Selection Tool you can limit your tonal adjustments to that area. If you only want to correct the background or foreground simply select that area and then open the Curves dialog box.
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