Image Resolution Basics for Large-Format Printing
ICT's Exhibits staff often receive questions about what quality and resolution photos and other images need to be for setting up and printing large-format posters, especially since digital photography has gained wider acceptance.
For the most efficient exhibit production, supplied images need to have a resolution equivalent to 100 to 150 dots-per-inch (DPI).
We urge digital camera users to shoot digital photos at maximum quality at all times. Always save the original file and work only on a copy of that file. Avoid saving JPG files in the JPG format again after editing and/or cropping the copy of the original. (See the following article, "File Formats: Pro and Con.")
So, What the Heck Does DPI Mean?
The terms and measurements used for resolution in desktop publishing, DPI, PPI, and LPI, can be a source of confusion.
One reason for confusion is that many software applications use terminology inconsistently. For example, many graphic applications (such as Adobe products) use PPI for their default resolution setting, while consumer-level software (such as Corel and many Microsoft applications) use DPI for image resolution designation. It's easy to see why people become confused.
DPI (dots per inch) is probably the most familiar but most misused measure of resolution. It's not the resolution of scanned images. DPI refers to the dots of ink or toner used per inch by an inkjet printer, laser printer, or other output devices that print text and graphics. Generally, the more dots per inch, the better and sharper the image should reproduce. The lower a printer's DPI, the less fine detail it can print and fewer shades of gray it can simulate. Think of DPI as the resolution of printer output for solid colors or solid black. Because monitor resolution is much lower than most printers, low-resolution images that look fine on-screen often print poorly.
PPI (or pixels per inch) is the number of pixels displayed in an image on the computer screen. A digital image is composed of samples that your screen displays in pixels. The PPI is the display resolution, not the printed image resolution.
LPI (or lines per inch) refers to the way printers reproduce images by basically simulating continuous tone images by printing lines of halftone spots. LPI is dependent on the output device and the type of paper used. To simulate shades of gray using only black ink, a printer prints varying sizes and patterns of halftone spots (spots are made up of many dots of ink or toner). Small halftone spots (using fewer dots) create the visual illusion of being light gray, while larger halftone spots (using more dots) appear darker. The number of lines per inch is the LPI, which is sometimes also called line frequency.
Garbage In, Garbage Out...
Remember, many tips and applications use the designations of DPI, PPI, and LPI interchangeably, so you need to look closely at the terms in context in order to figure out how they are applied.
In the meantime, here are some general tips to help you prepare your images for output:
1. If you start out with a bad image, you'll end up with a bad image. When you scan an image, its scanned resolution is the amount and type of information stored for that image. The better an image is to start, the more likely you'll get an acceptable printed image.
2. Remember that any re-sizing and/or re-sampling changes the stored information, so that the image resolution is different from the original scanned resolution.
3. When scanning photographic images, you need to know the final output method and size in order to be sure you scan at the proper resolution. Too much resolution results in wasteful and unnecessarily large file sizes. Too little resolution will result in lower-quality printed images.
4. Remember that there are no dots in an image until it is printed. (Note: Some scanners use SPI in place of DPI in listing their scanner resolution capabilities. The SPI designation is not as common as DPI or PPI.)
5. If you will be manipulating an image with graphics software (applying filters or changing color modes, for example), you may want to start with an image at a higher scanned resolution than what you actually need. Your file will contain more image data so you'll have more margin for error. Down-sample images to the required resolution after you edit or crop them, if needed.
6. As with scans, images acquired through digital photography, Web archives, or from CDs require the right amount of resolution for the final output method and size. Double-check their resolution and size by using Adobe Photoshop, if possible.
A Word About Using Web Graphics on Posters:
To display images from Web pages, monitors need to display images at 72 PPI, so less sampling is needed for on-screen display. Since images need to be down-sampled to 72 PPI to be used on the Web, resizing Web images larger to print on posters typically will not produce good final results.
Maximizing Image Quality
A big challenge for digital printing is reconciling image display size to the size of the final printed image. Throwing in re-sampling and re-sizing makes things even more confusing. With practice you'll be able to discern patterns and won't be quite so surprised when the image you see on-screen ends up printing at a different size or quality sometimes.
The lower the LPI, the more obvious the halftone dots will be in the printed image. High resolution image-setters can print a much higher LPI which result in smoother, almost-continuous tone photographs. A 300-600 DPI laser printer can usually only print at an LPI of 50-65, resulting in coarser images.