How to Use Curve Adjustment for Hard to Correct Photos in Photoshop CS6 - dummies

How to Use Curve Adjustment for Hard to Correct Photos in Photoshop CS6

By Barbara Obermeier

Photoshop CS6 offers tools that adjust curves in those tricky, hard to correct photos. The Curves command is one of the most advanced correction tools available, offering sophisticated control over the brightness, contrast, and midtone (gamma) levels in an image; control that’s far beyond what the Levels and Brightness/Contrast dialog boxes offer.

Whereas the Brightness/Contrast dialog box lets you change an image globally, and the Levels command allows you to change the shadows, highlights, and midtones separately, Curves goes far beyond either of those settings. It lets you change pixel values at 16 different points along an image’s tonal range.

Often, images that just can’t be fixed to your satisfaction with Levels can be helped with the Curves adjustment.

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You access the Curves dialog box by choosing Image→Adjustments→Curves or by pressing Ctrl+M (Command+M on the Mac).

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The following tips help you to begin understanding how to interpret the information and use the tools in this dialog box:

  • The horizontal axis maps the brightness values as they are before image correction (input).

  • The vertical axis maps the brightness values after correction (output). Each axis represents a continuum of 256 levels, divided into four parts by finely dotted lines. In the default mode, the lower-left corner represents 0,0 (pure black) and the upper-right corner is 255,255 (pure white).

    By default, the dialog box shows a 4-x-4 (quarter tone) grid; Alt-click (Option-click on the Mac) inside the grid to toggle it to a 10-x-10 (10% increment) grid. If you have the Curve Display Options visible, you can use the Quarter Tone or 10% Increment display buttons.

  • Whenever you open the Curves dialog box, the graph begins as a straight line. Unless you make changes, the input is exactly the same as the output, a direct 1-to-1 correlation.

  • You can expand the Curve Display Options and specify the following:

    • Show Amount Of: Choose between Light and Pigment to display the brightness levels or percentages. To keep things simple, leave them at the default of Light (levels), where darker values are at the bottom-left and lighter values are at the top-right. Choose between a simple or detailed grid (icons).

    • Show: If you’re adjusting curves for individual channels, choose Channel Overlays to superimpose those individual curves. Select Baseline to show your original straight line curve. Choose to display a histogram overlay. Select Intersection Line to display horizontal and vertical lines to help you align your points while you drag them on the curve.

  • The Eyedropper can show you pixel values. When you use the Eyedropper tool to click in the image, a circle appears on the graph to show you the value of the pixel being sampled. At the bottom of the Curves grid box, you can read the pixel’s input and output values.

  • Auto sets black and white values. When you click the Auto button, the darkest pixels in the image (the deep shadows) are reset to black, and the lightest areas are set to white. Like with the Levels dialog box, this option is the easiest way to make a correction, and has improved in CS6.

  • Eyedropper tools can also set black, white, and gray values. The Curves dialog box has black, white, and gray Eyedropper tools you can use to set the black, white, and midtone points, just like you can with Levels.

  • Select the Show Clipping options to have Photoshop display where clipping occurs in the image during your corrections. Remember, clipping occurs when a pixel’s values are lighter or darker than the highest or lowest value that can be displayed in the image, resulting in loss of detail in those areas.