Choose the Correct Photoshop Mode: Bitmap and Grayscale
Choose Image-->Mode to view image mode choices available in Adobe Photoshop CS5. Selecting the right one for an image is important because each mode offers different capabilities and results.
A channel contains the color information in an image. The number of default color channels in an image depends on its color mode. For example, a CMYK image has at least four channels — one each for cyan, magenta, yellow, and black information. Grayscale has one channel. If you understand the printing process, think of each channel representing a plate (color) that, when combined, creates the final image.
Bitmap mode offers little more than the ability to work in black and white. That’s it — no shades of color, not even gray. Most features are disabled in Bitmap mode, which is fine if you’re working on art for a black-and-white logo, but not for most images.
Many tools are unusable, and most menu options are grayed out in this mode. If you’re converting an image to bitmap, you must convert it to grayscale first.
Use Grayscale mode if you’re creating black-and-white images with tonal values, specifically for printing to one color. Grayscale mode supports 256 shades of gray in 8-bit color mode. Photoshop can work with grayscale in 16-bit mode, which provides more information, but may limit your capabilities when working in Photoshop.
When you choose Image-->Mode-->Grayscale to convert to Grayscale mode, a warning message asks you to confirm that you want to discard all color information. If you don’t want to see this warning every time you convert an image to grayscale, select the option not to show the dialog box again before you click Discard.
Using the Black & White adjustment is the best way to create a good grayscale image. Simply click and hold the Create New Fill or Adjustment Layer button at the bottom of the Layers panel and choose Black & White. Set the sliders to achieve the best black-and-white image and then choose Image-->Mode-->Grayscale.