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A GUIDE TO RESOLUTION...
One of the most potentially confusing things about working digitally is the aspect of resolution. Much of the confusion can be cleared, however, by familiarizing yourself with the relevant terms; what they mean and how they relate to each other. Once you do understand the terms, it's really just a mathematical process to achieve your desired result.
PPI: Pixels per inch
This is your display resolution, i.e., what you see on-screen. Usually referring to a monitor or digital camera. It also greatly affects file size and image efficiency - more on that later.
This is the most often encountered resolution term, and it alone can be confusing because the term relates not only to the on-screen display of any image, but also to the image file itself. To confound us even more, many people inaccurately use the term "DPI" in its' place, but don't be fooled. They usually mean "PPI" - "DPI" relates more accurately to printing an image (the description of DPI comes a little later).
*A side note: Whether you're on a Mac or PC does have some bearing here, since Mac screen resolution is 72 ppi, and PC screen resolution is 96 ppi. For PC users, exchange 96 ppi for 72 ppi in the math, and you'll get the result for your system.
First, let's address the way this term relates to the on-screen display of an image. In practical terms, a sample image 3" wide @ 150 ppi (file resolution) displayed on your monitor (screen resolution) at 100% size, is 6.25 inches wide.
3 inches X 150 ppi (file resolution) = 450 pixels. 450 pixels ÷ 72 ppi (screen resolution) = 6.25 inches.
Suppose the image was 3" wide at 300 ppi... If you do the math, you'll see that the on-screen display would be twice the width - 12.5 inches wide at 100%.
Note that both examples are 3" wide. The difference is increased resolution. If you print either image, you'll get an image 3" wide. The image with increased resolution will, however, print at a much better quality on an average or better printer. And that leads us to the next term...
DPI: Dots per inch
This is print resolution. Commonly referred to as density. This term is often misused when actually describing ppi, even though a pixel and a dot are not the same thing. Scanners will sometimes show a rating expressed as SPI: samples per inch. This is roughly equivalent to the dpi specification, although we are again talking pixel representations - not dots.
If you look at a printer for your system, somewhere in the description of the product there will be specifications on the printer's resolution. Expressed as dpi, an average example might be: Resolution: 600 X 600 dpi black, 600 X 300 dpi color.
The higher the quality of output available, the higher these numbers will be. Of course, no matter how high the printer's resolution is, an image file generated at too low a resolution will still print poorly. Later we'll discuss good practices for generating and scanning images for print.
LPI: Lines per inch
This is a halftoning resolution. LPI (also called linescreen) is most relevant to commercial printing. For example, the most common linescreen for commercial printing is 133 lpi.
When preparing an image for commercial printing, the surest way to get exactly what you want in print quality is to discuss the image and its' end use with your client, your printer or your service bureau. They will usually have a preferred set of specifications for their processes. They'll likely consider whatever their specifications are to be "standard," so the earlier in the process you have this discussion, the better. Often, there will be a full set of working procedures they recommend you follow, from the accepted software use to the colorspace to the final file format.
They may specify bit depth, and for DTP use, they may even provide you with a postscript printer description (ppd) or driver to set up your document for their printer. Save yourself some grief and cover this ground at the beginning.
These are general guidelines and should not be used to generate image output without first understanding when and why to choose each specification. After manipulation of any image, we recommend you archive a copy of the final document at full size and resolution.
General resolution guides
Web use: on-screen images. Scan or create your image at 150 ppi (more about scanning below). Convert the image to 72 ppi and save as your final web document.
Desktop publishing: laser or inkjet Scan or create your image in a multiple of your printer's resolution. If the printer resolution is 600 dpi, for example, create the image at a minimum (at full size) of 1.5 times the resolution - in this example, 900 dpi. You'll need to do some test prints from there to find how your printer performs best.
Commercial printing: consult your client, printer or service bureau. A rule of thumb is to create art at a minimum of 200 ppi for grayscale, 300 ppi for color at 100% of print size to be viable.
The best bit of advice we can give you here is this: use your scanner as it was designed to be used. Scan at multiples of your scanner's optical resolution; avoid interpolation for continuous tone images.
Interpolation increases resolution via resampling. This is a process of creating pixels to fill the enlarged areas of an image, finding averages for them based on the adjacent existing pixels and filling with those averaged tones. What you get is a larger image, but no detail is gained at all. In fact, the image will actually become fuzzy in greater interpolated resolutions. The only time interpolation is truly viable is in scanning line art. In that case, you can often achieve a good quality image at higher resolution. If you must increase the resolution of an image beyond your scanner's optical resolution maximum, you're going to get a better result by scanning at the optical maximum and resampling the image in a graphics program.
As long as you are scanning within the optical resolution of your scanner, it is a good idea to do as much of the adjusting of the image as you can within the scanner interface. It is more efficient, and therefore quicker to scan the image as close to your desired result as you can get it with the scanner's software.
So why not always scan or create an image at a high resolution?
Too high a resolution is inefficient from the creation of the file through adjustment and manipulation all the way to end use. That is a very costly error, so scan, work and output at resolutions that are suitable for the end use.
Do you illustrators out there notice anything familiar? Working at 150% final size in commercial work... you ought to be comfortable with that formula.
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