Frequently Asked Questions :
How large should my digital files be?
Why convert and work in RGB instead of CMYK?
How do I color balance my image in Photoshop?
What is saturation?
How do I properly sharpen an image in Photoshop?
If your image has been scanned or digitally photographed with a standard grayscale swatch, you can check that the RGB values are balanced. Each of the values R, G, and B of the swatch should be nearly the same, thus reflecting a balanced gray.
Zoom in to the area containg the swatch. Set your sampling tool optionsto an area of 5x5 pixels. Hold the cursor over the white box of the swatch. The RGB values should be approximately 250, 250, 250, almost a pure white. Note that the Info tab shows values separated by a slash, these reflect the current and new values of the adjustment.
If there is much variance, adjust each channel independently by using the far right sliders in Levels. Now check a box near the dark end of the swatch. It should read something near 20, 20, 20, almost pure black. Adjust levels using the far left slider. Finally, check the middle boxes of the swatch and adjust these with the top center slider in Levels.