Image Sizing for Printing

While resizing image files for monitor display or screen projection can be done at a relatively low resolution of around 100 pixels per inch (ppi) with good sharp results, reproduction as prints requires much higher resolution. It is a phenomenon that monitors can display an image very well at a lower resolution, but prints made from the same low res files are likely to be disappointing.  A printer needs to have more pixels in the image files sent to it.

The generally accepted resolution for printing is 360 ppi for best results.  Acceptable results may be obtained at 240 ppi, but experts, such as Scott Kelby of National Association of Photoshop Professionals and prolific author of “how to” books, recommends at least 300 ppi. Costco specifies that resolution for images sent to their printer.

Thus, when you resize image files to send to your desk top printer, use that resolution and the desired dimensions in inches when processing your image.  Depending on the intended size of the print to be made and the original file size, this may require an enlarging step. For a wide carriage paper size of 13” x 19” this will mean an increase from the typical 12 megapixel camera, since its “native” dimensions at that resolution would be smaller.  Specifically, in my Nikon D 300 the native dimensions are 4288 x 2848 pixels. Dividing each dimension by 360 ppi yields a native size of roughly 12 x 8 inches.  The increase in area size to 13 x 19 inches is roughly 160 percent.  However, the file size will jump from a 12 MB NEF file to about 70 MB in TIFF format, a fivefold increase.

While the enlarging step involves the creation of new pixels, Photoshop Elements, Photoshop, Lightroom, and other programs use a method of sampling the neighboring pixels in order to select and create the appropriately colored pixels to achieve excellent results in the enlarged print. In the earlier days of digital photography, there was a myth that one should never exceed the native resolution when enlarging prints, but it was just that, a myth. 

Once enlarged to the desired dimensions, your file is ready to send to the printer.  Ink jet printers lay down micro droplets or dots and the resolution is measured in dots per inch, or the more commonly used term, dpi.  Unfortunately, these two terms, ppi and dpi, while unrelated, are often used interchangeably, even by those who should know the difference.  It is important to remember that ppi refers to pixels per inch in camera/computer generated image files, while dpi refers to physical dots of ink on paper. There is no correlation.

Your desk top printer will have a recommended print resolution depending on the type of output. This may be in a specific dpi or simply the quality level, such as Photo. For my Epson 2880 the recommended level is 1440 dpi. (While 2880 is possible, it runs slower, uses more ink and the results are not easily detectible, so it is not recommended).  Use it and have fun.

Questions? Contact Bob Coffey at Bobcoffey@aol.com or 828-595-5000.