Level 3

Lesson 18

Learning how to make tonal adjustment to a dark image using the Curves command

Introduction

Many Curves adjustments can be reproduced by using Levels. However, Curves command provides up to 14 control points for highlight, midtone and shadow adjustments, so you can perform very precise correction.

The following lesson will teach you how to adjust tonality to a dark image by using the Curves command.

·   Open an image file.

To help us understand the overall contrast in this image.

·   Go to the “Menu bar”.

·   Choose Window > Histogram

The “Histogram” palette appears.

The histogram shows that pixel values are concentrated on the left end, identifying that this is a dark image.

·   Go to the “Menu bar”.

·   Choose Image > Adjustments > Curves

The “Curves” dialog box appears.

·   Check the “Preview” box so that we can see the effect of changes in the image.

By default, Curves is set to brightness mode for RGB images.

To find out which mode is in use:

·   Check the horizontal bar under the graph. When the dark end of the bar appears on the left, we are working on the brightness mode.

If the dark end of the horizontal bar appears on the right, we are on percentage mode.

Notice: In percentage mode, the curve moves from highlights on the left (0%) to shadows on the right. (100%).

·   Click the double-arrow on the bar to reverse back to brightness mode.

If we are using the brightness mode already. No need to do this step.

In brightness mode, the horizontal axis of the graph shows the original brightness values of the pixels (input levels); the vertical axis denotes the new brightness values (output levels).

The curve (now a diagonal line) moves from shadows on the left (0) to highlights on the right (255).

·  Reposition the Document window, the Curves dialog box and the Histogram palette by dragging their title bars, so that we can see all of them simultaneously.

On the “Curves” dialog box:

By default, the Point tool is selected.

·   With this tool, click to add a control point to the midtones, a point to the 3/4 tones and a point to the 1/4 tones of the curve.

Notice: We can add up to 14 control points to the curve to compel the curve to stay fixed, and then drag between points to make precise adjustments.

To remove a point from the curve, drag the point out of the Curves dialog box.

·   Drag up the entire curve by moving up the three points.

Notice: Move up in the graph for higher output values; move right for higher input values. Note how the Input and Output values change following the location of the pointer in the graph.

·   Look at the “Histogram” palette.

As we drag the entire curve up, pixels redistribute to widen the tonal range of the image.

·   Look at the “Document window”.

The image is lightened.

Go to the “Curves” dialog box again.

·   Adjust the shape of the curve while watching the effect on the image. Stop when we get the right effect.

Notice: Move up a point near the top of the curve will lighten highlights; move down a point near the bottom of the curve will darken shadows

·   Look at the “Histogram” palette.

After the shape of the curve is adjusted, the histogram expands to a full tonal range with a number of pixels in all areas.

·  Look at the “Document window”.

The lightness and contrast of the image is improved.

Conclusion

We learned the difference between brightness mode and percentage mode.

We learned that, on brightness mode, dragging up the entire curve will lighten a dark image.

Finally, we learned that we can control the lightness and contrast of an image by adjusting the shape of the curve.