About alpha channels (Photoshop)

An alpha channel has these properties:

• Each image (except 16-bit images) can contain up to 24 channels, including all color and alpha channels.
• All channels are 8-bit grayscale images, capable of displaying 256 levels of gray.
• You can specify a name, color, mask option, and opacity for each channel. (The opacity affects the preview of the channel, not the image.)
• All new channels have the same dimensions and number of pixels as the original image.
• You can edit the mask in an alpha channel using painting tools, editing tools, and filters.
• You can convert alpha channels to spot color channels.