Preparing Your Files for Best Results II:   

How DPI, PPI, and MP Affect the Size and Quality of Images for Print and Digital Media   

See how ink dots make up the photo

DPI (Dots per inch) refers to the number of dots of ink or toner per inch used by a printing device to produce your text and graphics. The number of dots of ink are what make up the printed images that we actually see. If you look through a loupe at a photo in a magazine, you will see how the dots of ink make up the picture. More dots per inch result in a sharper image. What makes this confusing for the average person, is that while it is not the measure of the resolution, the term DPI is often used incorrectly and interchangeably with the term PPI.  

Not to confuse you more, but DPI is relevant in commercially printed pieces in combination with LPI (Lines per inch) to determine the resolution of an image. LPI is dependent on the press on which the piece is being printed. Your designer or print shop will ask you to provide high resolution photos and will know how to edit them to get the best results.

Low resolution

High resolution

PPI (Pixels per inch) is
a measurement of resolution that defines the size at which an image will print. The higher the number of pixels, the larger the physical size of the image. For example, a photo at 448 x 336 pixels is 6.22" x 4.667" photo at 72 PPI. That works fine for use on your web site. But for commercial printing the photo needs to be 300 PPI. So, that same photo converted in an image editing program to 300 PPI is only 1.5" x 1.12" - too small. It is always best to start with a higher resolution (PPI) image, because it can be reduced while maintaining its quality; a low resolution image loses quality when it is enlarged. When a 72 PPI image is converted to a 300 PPI image, it loses approximately 75% of its physical size. The photos above show the difference between correctly and incorrectly sized photos.  

That brings us to the next topic, MP (Megapixels).   

Set to 5 MP or higher

MP (Megapixels) refers to resolution as it relates to your digital camera. If you are taking photos that will be used for marketing purposes, I always suggest that you set your camera to a minimum of 5 MP. That way you will have a photo that can be used for print applications, and reduced for web applications. A photo taken at 5 MP results in a photo that is 2560 x 1920 pixels at 72 PPI or 35.6 in. x 26.7 in. The same photo converted for printing purposes to 300 PPI results in an 8.5 in. x 6.4 in. When you enlarge a photo, your software attempts to add pixels to fill the spaces and that can result in a serious loss of quality (see high and low resolution photos above). 

Bottom-Line: DPI refers to the resolution of a printing device; PPI refers to the measurement of resolution that defines the size of the image; MP is the resolution at which your camera takes photos.

An image can always be reduced from a large size and maintain its quality; but when enlarged from a low resolution file, will end up looking jagged or fuzzy. Better to down-size from a high-res file than to up-size from a low-res file.