If you plan to print your image using a
halftone screen, the range of suitable image resolutions depends
on the screen frequency of your output device. Photoshop can determine
a recommended image resolution based on the screen frequency of
your output device.
Note: If your image resolution is more than
2.5 times the screen ruling, an alert message appears when you try
to print the image. This means that the image resolution
is higher than necessary for the printer. Save a copy of
the file, and then reduce the resolution.