Slightly contrary to the previous answers, the resolution for large dimension file output (for instance billboards), CAN often be LOWER than the 300 ppi resolution required for general offset printing. It depends on the final intended usage. BTW, there is a big difference between PPI (pixels-per-inch) and DPI (dots-per-inch). Most folks have commonly come to use DPI when they actually mean PPI, it's kind of like how we all call facial tissues "Kleenex", it is a commonly accepted part of our dialect, but basically it is misleading. (I kind of cringe internally whenever I hear someone say DPI when I know that they mean PPI, but alas, that is something I have no power to change, except by "one student at a time"!)
PPI actually means "how many pixels will fit in a linear inch inside your digital file". DPI means "how many dots of ink a printing device can lay down within a linear inch on the printed paper". There is an intimate relationship between the two for sure, but they are definitely NOT the same thing. As Stacy and Joyce suggested, it is important to ask the print vendor what PPI they need for the file you plan to submit, taking into consideration your intended final output size, their printing device's DPI resolution, and the intended viewing distance. Additionally, your file dimensions are a critical aspect too. If you are designing with a 3 feet high by 5 feet wide FILE size, but you want the final PRINTED OUTPUT to be double that size at 6 feet high by 10 feet wide, you need to DOUBLE the resolution that they tell you, because when they enlarge your 3x5 to 6x10, the number of pixels remains the same, they just spread out over a greater distance, and your 100 PPI file will have become a 50 PPI file (in this example), which has now become half of their minimum requirement. In this enlargement scenario, if they asked for 100 PPI, you will need to make it 200 PPI. When they double the output size of your 3x5 at 200 PPI file, it will become 6x10 at 100 PPI, right on their target resolution.
The reason "final intended usage" comes into play is a function of the ability (or lack thereof) of the human eye to resolve (or discern) the "dots" on the printed piece. For instance, if a billboard vendor requires only 100 pixels per inch in your file, they know that your file resolution (when combined with their print device's DPI capability) will be adequate because most viewings of that billboard will be at a physical distance that is too far away for the human eye to discern the individual dots on the printed piece. Billboards generally print at a measly 15 DPI, which means that each printed dot is approximately one sixteenth of an inch in diameter (ginormous dots)! Therefore, a number of blue (cyan) dots interlaced with a number of red (magenta) dots when viewed from that longer viewing distance will appear to the human eye as a shade of purple, and the size of each dot and their distribution on the printed page (as well as the amount of white paper showing through around those dots, where no ink has been laid down) will determine exactly what shade of purple you perceive. And sometimes yellow and black, and occasionally other ink colors are added to the mix to extend the range of purples that can be printed (keep in mind that printing costs might go up whenever you are adding ink colors beyond CMYK). On the other hand, if you were to climb up on that platform where the installer goes to put up the billboard printed piece and viewed the printed piece from a distance of 1 to 5 feet or so, at 100 PPI file resolution printed at 15 DPI you will definitely be able to see the individual red and blue dots sitting next to each other (but you might NOT be able to perceive the overall image anymore!) Similarly when you see fog outside, you cannot discern the individual water droplets, but when you look into the fog, you are seeing millions of these droplets at once, and they can be perceived as "fog". Higher concentrations of vapor molecules in line with your gaze move the fog towards opacity, lower concentrations move the fog towards transparency. You can almost always discern both states in the same fog bank, just as you can perceive different shades of purple in a printed image. (A slight difference with this simile: water vapor molecules are pretty consistent in their size, printed dots can vary in diameter).
As another example, take a look at a color photo in any modern color-printed magazine. Usually it will look like a continuous tone photograph, right? (BTW, photos from a photo lab don't use "dots", they employ continuous tone emulsions that actually blend the colors together, unless they are using inkjet, or Giclee technology, and if you want to know, you'll have to ask). Now look at that printed magazine photo again under a high powered magnifier, and the dots will reveal themselves! At a normal reading distance, your eyes cannot discern these individual printed "dots", and they blend together to show you a wide range of different colors, even though most print jobs only contain four distinct ink colors (commonly referred to as CMYK, where C=cyan, M=magenta, Y=yellow and K=black). Combinations of the printed "dots" of these four ink pigments, their relative sizes and their distribution in relation to each other all work together to provide what we all call "full color printing". In a way, it is a clever illusion! Imagine that magazine image at one inch wide and at 15 DPI, the picture would not be recognizable as anything but a bunch of colored dots, with a total of 15 dots across that horizontal linear inch ("purple" would be a combo of at least alternating cyan and magenta dots). In reality that magazine image has been reproduced with a DPI of somewhere between 240 to 300 DPI (approximately 20 times smaller dots), and those individual dots, being too small to be discerned by our naked eyes blend together and essentially trick our eyes into seeing a "full color image", with a wide variety of tones and values.
Further supporting info:
- The diameter of a printed dot at 15 DPI is 0.066 inches across (around one sixteenth of an inch, or 66 THOUSANDTHS of an inch).
- The diameter of a printed dot at 300 DPI is 0.00333 inches across (33 TEN-THOUSANDTHS of an inch).
- A dot that small is essentially too small to see. Scientists generally agree that the smallest individual objects we can see are 0.00393 inches across.
Still baffled by this complex topic? Consider scheduling some sessions with me!