In this introductory column of Photoshop Basics, we are going to define the concept of resolution and try to put to the bed the myths about which resolution is best for each situation.

Defining Resolution

There are many terms thrown around when discussing resolution--dpi, ppi, and lpi. First, let's shed a little light on what these acronyms actually mean.

dpi = dots per inch	This term is used mainly with laser and inkjet printers, and measures the amount of ink in an inch of printed output. If your printer is capable of 1440 dpi should you print your images that high? Not really, since you'll find that 150 dpi to 200 dpi is all you really need to get quality prints. As always, experimenting is the best way to find the optimal settings for your printer, but bear in mind higher resolution leads to higher file size.
ppi = pixels per inch	Pixels are also known as picture elements. Pixels, simply put, are blocks of color in a grid. As you zoom in on an image you can see the actual pixels in the image. An image that is 72 ppi by 72 ppi, basically translates in inches to 1 by 1. This is the preferred way to discuss resolution in a digital file. Almost all image editors on the market today use pixels as their unit of measure when discussing resolution.
lpi = lines per inch	Lines per inch is used in the commercial printing industry. In common terms, lpi is the number of lines of ink that an image takes up on the plates of a printing press.

How do these measurements relate to one another?

• dots per inch (dpi) and pixels per inch (ppi) are basically the same concept, but they are used to measure different mediums--dpi for printed output, ppi for digital media.
  
  
• lines per inch (lpi) is roughly equivalent to half of ppi and dpi. For example, if you have a ppi of 300 then your lpi would be 150. Files that are being outputted via this method tend to fall in the range of 150 ppi to 300 ppi.

You have acquired an image, now what?

Determining what resolution to use depends on what your output will be, but it is always better to start with a higher resolution scan and then sample down to the output size you need. Use the following as a guideline:

For any screen-related medium the image should be low resolution.
Images for the web should be 72 ppi, since smaller file size is the name of the game.

If your final output device is a computer screen, it's better to start with a higher resolution scan (say 150 ppi) and then edit the final image down to 72 ppi or 96 ppi. For example, PowerPoint onscreen shows should be 72 dpi or 96 dpi depending upon platform playback.

You are probably wondering why either 72 ppi or 96 ppi? Well, the 2 major computer operating systems (Macintosh and Windows) measure screen resolution differently. The Macintosh operating system assumes a default screen resolution of 72 ppi while Windows assumes 96 ppi.

Last, but not least, I would be remiss if I didn't mention the other screen medium--television. Now you may wonder why you would ever need to output anything to a TV? Well you never know, so why not be prepared anyway? Television is a low resolution format. Here in the US we use the NTSC format. Television's default resolution is 640 pixels by 480 pixels at 72 dpi or, if outputting to television in Europe (the PAL format), the default resolution is 720 by 576.

For images that will be printed on paper use higher resolution (in the range of 150 to 300 dpi).
For example, if you have Medical Media Services design a poster for you, the large format printer they use has a resolution of 150 dpi.

Hopefully this has cleared up any resolution dilemmas you have. If not, feel free to direct comments to me at: sean.dyer@jefferson.edu.