Two Numbering Systems

Two different numbering systems can be used in the Curves dialog box. You can switch between the 0–100% system and the 0–255 numbering system (which we used in Levels) by switching between ink and light (remember the little arrows that appear in the middle of the bottom gradient). Go ahead and give it a try (Figure 6.58).

There are two main causes for undesirable results: making the curve flatten out (become horizontal) or making the curve go downhill (when moving from left to right). You can often fix these problems by adding a point in the middle of the problem area (the flat or downhill part of the curve) and then moving that point so that it appears next to the point that is furthest away from the area that was causing the problem. The idea is to finesse the position of the point so that it appears right next to one of the other points and in is a position that prevents the curve from becoming flat or going downhill. Clicking the Save button brings up a standard Save dialog box, which allows you to save the current settings for future use. The file that's created contains the Input and Output settings for each of the points used on the current curve. To reuse a saved setting, click the Load button. When you're changing the Input and Output numbers, press the up or down arrow key to change a number by 1, or press Shift–up arrow or Shift–down arrow to change a number by 10.

If you're working on an image that's in RGB mode, Photoshop assumes you're going to use the image onscreen instead of printing it. Therefore, when you open Curves, it uses Input and Output numbers ranging from 0 to 255. These numbers represent the amount of light your monitor will use to display the image onscreen (0 = no light, or black; 255 = maximum light, or white). Using this numbering system allows you to have control over each shade.

If you're working on an image that's in grayscale or CMYK mode, Photoshop assumes you'll be printing the image. Therefore, when you open Curves, it uses numbers ranging from 0% to 100%. These numbers represent the amount of ink used to reproduce each level of gray in the image (0% = no ink; 100% = solid ink).

When you click the arrows that switch between the two numbering systems, Photoshop also reverses the gradients at the bottom and left of the graph. It does this to keep the zero point of each numbering system in the lower-left corner of the graph, which effectively changes between light and ink. You don't have to remember or understand why this happens—it's just nice to know there's a reason behind it.

When you switch the numbering system, this also changes the gradient on the left side of the Curves dialog box. Therefore, if you're using the 0–255 numbering system, you have to move a curve up to brighten the image and down to darken it (the exact opposite of what you do in the 0–100% numbering system). I always look at the gradient on the left to remind me: If black is at the top (the 0–100% system), you're using ink, and moving a curve up will darken the image. If white is at the top of the gradient (the 0–255 system), you're using light, and moving a curve up will brighten the image.