Viewing distance plays a major role in determining the required resolution. An alternate method to determine the best image resolution is to first determine how far away the average viewer will be viewing your print. You can reduce the resolution by a third for every 6 feet we move away from a print.
One method to determine the optimal image or base resolution required for an image file in large-format printing is to divide the output resolution by three. For example, if an output device can print at 360 dots per inch (dpi), 120 dpi is the required image or base resolution for your file printing on a device that prints at 360 dpi.
For example, starting with 360 dpi as maximum resolution, an image to be viewed from 1.8 m (6 ft) away could be printed at 240 dpi. If the viewing distance were 3.6 m (12 ft), however, the image’s resolution could be lowered to 120 dpi, be- cause the human eye cannot see 360 ppi of detail from 3.6 m (12 ft) away.