PPI - Display Resolution

PPI (pixels per inch) is how a monitor displays an image. How an image looks on screen is determined by the resolution of the monitor — the number of pixels it can display in a given area. PPI and SPI are frequently used interchangeably as a measure of image resolution, as most Adobe Photoshop users will know.

Although Real World Scanning and Halftones and some other sources make a distinct separation between samples and pixels, in practice it is probably not reasonable to expect most users to trade in one term for the other. However, for the purposes of this article, for the purists, and for the benefit of the curious, PPI does differ from other terms of resolution. It may also be the failure to differentiate the terms that leads to the greatest confusion over how an image looks on screen and how it looks when printed.

How SPI, PPI, DPI, and LPI relate
PPI is a function of your monitor. A pixel or "picture element" is a specific x,y coordinate (dot) on your screen. A high-resolution setting displays more pixels per inch. In practical terms, the same picture on a low resolution monitor looks larger than it does on a higher resolution monitor because the pixels are spread out over a larger area.

One of the biggest challenges new (and even seasoned) desktop publishers face is reconciling image display size to the size of the printed image. Throw in resampling and resizing and it really gets confusing. Here's a little exercise I performed and the results. I freely admit that it confuses the heck out of me too but at least you know you're not alone. With practice and patience you'll be able to discern a pattern and won't be quite so surprised when the image you see on screen prints at a radically different size some of the time.

The following exercise refers to the illustration in the sidebar: Scanned a 1" x 1" (physical size of the photograph) image at 300 SPI Downsampled the 300 SPI image to 96 SPI at 1" x 1" (96px x 96px) Downsampled the 300 SPI image to 96 SPI at 3" x 3" (300px x 300px)

The first thing you might notice is the difference in size of the images. Why do image one and two appear so different in physical size when I only changed the resolution? Because the monitor displays in pixels and 300 pixels per inch takes up more screen space than 96 pixels per inch. (And they would take up differing amounts of screen real estate if your monitor is set to 640 x 480 or 800 x 600, etc.) So what happened with image 3? It's only 96 SPI but its physical size is 3" x 3" (300 pixels square) so it displays at the larger size.

So, do you want to guess what the printed images will look like? The first two (large 300 SPI and small 96 SPI) printed at the same 1" x 1" size. (The 300 SPI image looked better though). The large 96 SPI image with a physical size of 3" x 3" printed at 3" square.

As you can see, how it looks on screen can be quite different than how it prints. Check the actual dimensions, as well as resolution of your image, to know how it will print.

In practice, SPI and PPI are often used interchangeably. DPI is frequently used in place of one or both terms. However, even if you call it DPI, remember that each dot or "unit of measure" behaves differently depending on whether it is a scanner (or scanned image), a monitor (or on-screen image), or a printer (or printed image).

Next > DPI: Printer Resolution

Measurements Systems > Measuring Resolution > PPI: Display Resolution

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DPI - Printer Resolution

DPI (dots per inch) is probably the most familiar and most misused measure of resolution. For desktop publishing, it is probably the least important measure as well. It is not the resolution of scanned images. It is not the resolution of images on screen. It is the measure of how many dots of ink or toner a printer can place within an inch (or centimeter).

Most printers print the same number of dots horizontally and vertically, though some may have differing numbers. Basically, 600 dpi printers print 600 tiny little dots across one inch and 600 dots vertically for one inch.

How SPI, PPI, DPI, and LPI relate
The lower the DPI of a printer, the less fine detail it can print and the fewer shades of gray it can simulate. Because monitor resolution is so much lower than printers, low resolution images that look fine on-screen often print poorly.

DPI, Scanning, & Digital Images: DPI is only relevant in that the combination of your printer's DPI and the required LPI determine how much resolution your image must have. If you will be printing to a 300 DPI printer, resolution in excess of 300 SPI is rarely necessary.

DPI and Printing: In general, the higher the resolution of your printer or imagesetter, the greater detail you can print and the better appearance of your output. See Part 5: LPI for more on printing images.

DPI and the Web: On the Web, DPI is irrelevant because images display on-screen in pixels (PPI).

In practice, SPI and PPI are often used interchangeably. DPI is frequently used in place of one or both terms. However, even if you call it DPI, remember that each dot or "unit of measure" behaves differently depending on whether it is a scanner (or scanned image), a monitor (or on-screen image), or a printer (or printed image).

Next > LPI: Halftone Resolution

Measurements Systems > Measuring Resolution > DPI: Printer Resolution

View the original article here

Scanning Resolution - Scan Your Images At This Resolution

The best resolution to scan your images depends on how it will be used - on screen or in print. For the purposes of this article, we'll assume the image is for print. To find your best scanning resolution you need to calculate the final image resolution, that is, the resolution of your image when placed in your file for printing. Your scanning resolution and final image resolution may be the same, but not necessarily. For most users, it is rare that you will need thousands or even more than 200-400 pixels of resolution.

To reach your final image resolution, you need to know 5 things before scanning:
Size of the original image.
Are you scanning a 4x5 inch photograph, for example? Size of the printed image.
Will your image be reduced or enlarged? LPI at which your image will be printed.
LPI depends on the printer you are using. The type of paper, also affects LPI. Your final image resolution is usually about double the LPI at which the image will be printed. Resolution Multiplier.
This number is usually 2. You can see how it is used below. For most purposes, 2 is the magic number. Editing of the scanned image.
It is often better to scan at a somewhat higher resolution than you'll eventually need to allow for pixels lost during image editing. Just remember, the higher the resolution, the larger the file size. Don't go overboard.

For commercial printing, if you don't know the required LPI, talk to your print shop. They can help you come up with the right numbers based on the specifications of your print project. Here's a quick reference list or see the more detailed chart in the Resolution Inch by Inch article on this site:

To calculate your final image resolution:
If the size of your image will not change, multiply LPI by your resolution multiplier. LPI x 2
For example, an image destined for a brochure on uncoated paper using offset printing needs a final resolution of approximately 133 x 2 or 266. If the size of your image will change, multiply LPI by your resolution multiplier and then by the size of your final image as a percentage of the original. LPI x 2 x %
For example, if our image for the brochure, above, is to be enlarged by 25%, then it needs a final resolution of 133 x 2 x 125% or 333. If it is to be reduced by 25%, then it needs a final resolution of 133 x 2 x 75% or 200. (I've rounded up in each case.) Don't know the percentages? Use the scanning calculator, below.

You may be fine scanning at the final resolution arrived at by one of the formulas, above. However, since most scanned images need some type of image editing, scanning at a slightly higher resolution is often advantageous. It is easier to "throw out" unneeded resolution after scanning than it is to restore lost resolution after the fact. Just remember to reduce your image to the final resolution before placing it in your document for printing.

Having trouble doing the math? Try this simple scan resolution calculator:

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