How do I know when I need to use interpolation?

A common publishing standard is to use images that are 300dpi or greater.  Large prints that are typically viewed from a distance are sometimes printed at 200dpi with satisfactory results.

 

Given the 300dpi standard, divide the image’s pixel height and width by 300 to yield print size in inches.

 

For example, the 5 megapixel Olympus c-5050z captures images at a resolution of 2560×1920.  Dividing 2560×1920 by the common publishing standard of 300dpi yields a maximum print size of 8.5″x6.4″ (2560 / 300 x 1920 / 300).

 

Prints significantly larger than 8.5×6.4 would likely benefit from interpolation.  It really depends on the personal taste of the viewer.  An 8×10 image size would be roughly 240dpi which would probably look just fine.

 

200dpi is often used for prints that will be viewed from a distance.