So, when printing a grayscale image with black ink you really are printing a B&W photograph because the halftone is simply black dots of ink.
You can produce digital halftones direct from the software to the printer. The halftone effect used may be specified in your printers PPD (PostScript Printer Driver) or set specifically in your software program.
When printing B&W photos to an inkjet printer, results can be varied by printing with black ink only or allowing the printer to use color inks to print the shades of gray. Color shifts -- from negligible to obvious -- may occur when using color inks. However, black ink only can lose some of the finer details and result in more obvious dots of ink -- a more noticeable halftone.
For commercial printing, leave grayscale images in grayscale mode unless your service provider suggests otherwise. Depending on the printing method, the black and white halftone screens are much smoother than what some desktop printers can acheive. However, you can specify your own screens in your software if you prefer (or to create special effects).
See "The basics of color and black & white halftones" for more on working with halftones.
