Down to Brass Tacks Let's cut the crap.

Resampling images for print

Regardless of the time an image takes to be spooled to the printer, resampling an image (i.e.: changing its pixel size using interpolation) is to be avoided, as resampling an image is detrimental to its quality.

The bottom line is this: if the native size of the image allows you to print using a resolution between 180 and 480 ppi, and if proper sharpening has been applied (considering the destination of the print, its resolution, etc.), you should let the printer do whatever interpolation is required.

Only if you intend to make a print so large that you have less than 180 ppi available, or if you intend to make a print so small that the resolution is above 480 ppi, should you resample the image. If you need to resample, keep this in mind:

• It is arguably preferable to resample by doubling (200%) or halving (50%), rather than using intermediate steps.
• For up-resing, the preferred algorithm is "Bicubic Smoother" — although you will then have to add a significant amount of sharpening.
• Output sharpening should always be applied after resampling.

For a detailed discussion on the topic of sharpening, refer to the Real World Image Sharpening book series. The Luminous Landscape also has video tutorials on the topic, notably chapter 12 of "From Camera to Print".