Pick Your Scanning Resolution
Calculate the Best Resolution for Your Scans
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:
| Related Resources |
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• Resolution Basics for Beginners • Resolution in Detail - SPI, PPI, DPI, LPI • Scanning Resolution, Scaling, File Size |
- 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.
| screen printing | 35-65 |
| laser printer/photocopier | 50-110 |
| offset printing | 60-150+ |
| standard for newsprint | 85 |
| standard for offset printing | 133 |
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.
Having trouble doing the math? Try this simple scan resolution calculator:
Having trouble doing the math? Try this simple scan resolution calculator:
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Size of the printed image:
WIDTH: HEIGHT: |
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LPI at which your image will be printed: |
Resolution Multipler (2 is standard, for most uses no need to change this number): |
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The image will be enlarged/reduced by: percent Your scanning resolution should be at least: samples per inch | |
Getting Started Scanning...
• Introduction
• Part 1: DPI for Email & Web
• Part 2: Saving Your Scans
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The information here pertains to both color and grayscale images that will be printed on an inkjet printer. It does not apply to printing line art images. For more discussion on printing line art images see Scanning Line Art by Desktop Publishing Guide Jacci Howard Bear. (Continued below...)
PPI vs. DPI
The first important point to understand is the differences between PPI (pixels per inch) and DPI (dots per inch). Many software programs and scanner interfaces use these two terms interchangeably but that's not exactly accurate. As a general rule, and for the purposes of this article, the term PPI should be used when referring to image resolution, and the term DPI should be used when referring to printing resolution. How can you remember this? Monitors display pixels, and printers produce dots. So, as you follow this discussion, whether your software calls it DPI or PPI, when I talk about the resolution of your image, I will be using the terminology PPI.
Pixel Resolution
Digital photos and scans are all bitmap graphic types, no matter what format the image is saved to. That means they are made up of a grid of individual, tiny pixels with each pixel representing a single color in the image. If you zoom in on one of these images (see example) in your imaging software you'll be able to see the individual squares of color.
What this means in relation to printing is that these types of images are resolution-dependent. In other words, the quality of the print and the size of the printed image is limited by the number of pixels in the image. You can't increase one value without effectively decreasing the other.
Inkjet Printer DPI
Today's inkjet printers have three standard output settings:
normal:
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300 x 300 or 320 x 320 dpi |
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high quality:
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600 x 600 or 720 x 720 dpi, 1440 x 720 |
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photo quality:
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1200 x 1200, 1440 x 1440 dpi, 2880 x 1440 and up |
You might also have a draft or economy setting, but you should never use this setting for printing images. It's primarily used for printing text and rough drafts.
A popular myth is that it's necessary to scan an image at the same resolution that you will be using to print. In the case of color and grayscale images, this only results in excessive file sizes. At one time, a good general rule for inkjet printing was that you needed half to one-third of the PPI of the printer's DPI setting that you intend to use. So if you're using your printer's "normal" setting (300 dpi), your image needed to have at least 150 ppi. When using the higher quality printer settings (720 dpi and up), you can bring the PPI down to about 1/3 of the output resolution. That would be 240 ppi for your printer's 720 dpi setting.
Since writing this article photo printers have gotten much better--the dots are smaller and more compact--but still, you rarely need your image resolution to be higher than 240-300 ppi for inkjet printing.
Calculating Image Size and PPI
So how do you calculate the image size you need? It's simpler than you might think. First, decide what size you want your printed image to be, then multiply the height and width in inches by the PPI you need for the print resolution you'll be using. Here's a reference chart for some common print sizes:
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Printer Quality
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Draft/Economy
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Standard
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High/Photo
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Print @
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300 - 320 dpi
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600 - 720 dpi
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1200 - 2880+ dpi
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Scan @
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150 ppi
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150-240 ppi
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240-360 ppi
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Printed Size
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Actual Pixel Dimensions (Average)
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2" x 3"
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300 x 450 pixels
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400 x 600 pixels
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600 x 900 pixels
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4" x 6"
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600 x 900 pixels
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800 x 1200 pixels
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1200 x 1800 pixels
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5" x 7"
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750 x 1050 pixels
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1000 x 1400 pixels
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1500 x 2100 pixels
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8" x 10"
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1200 x 1500 pixels
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1600 x 2000 pixels
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2400 x 3000 pixels
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Why is my scan so HUGE?
Keep in mind that when you scan an image at anything higher than 72-100 ppi, it's going to display much larger on your monitor than the original picture or page. That's because your monitor can only display a fixed number of pixels per inch, usually 96. When you place the image into a page layout program, most software will be able to read the PPI information and will place your image into the page at the proper size. Some of the low-end printing software may not, however, and you'll have a very large image placed into your page. When that happens, you'll have to size the image in your layout program to the printed size you based your calculations on.
But I don't know what the output will be!
Whenever you increase or decrease the pixel dimensions of an image you lose some quality, but it's less destructive to reduce pixel dimensions rather than increase pixel dimensions. If you're scanning for archival, or you don't know what the intended output will be, it's better to scan at a higher resolution and reduce the image later. However, it is of no advantage to scan higher than your scanner's maximum optical resolution.
PPI and Digital Cameras
When you're printing images from a digital camera, you have to calculate things a little differently. With a digital camera, you have a fixed number of pixels which is the maximum your camera is able to capture. Today's lower-priced cameras have an average maximum resolution of 1200 x 1600 pixels (2 Megapixels). In this case, we have the pixel dimensions, so we need to work in reverse to figure out the best quality setting and printed size.
Using the formula above we can calculate that with 1200 x 1600 pixels we can either get a normal quality print at 8 by 10 inches, a high quality print at 5 by 7 inches, or a photo quality print at 4 by 5 inches maximum. That's not a very big image for a photo-quality print, so if you thought you'd be producing photo-quality 8 by 10 prints with your $200 digital camera and inkjet printer, you're probably going to be somewhat disappointed. As you can see, there is a choice to be made as to whether you want to sacrifice printed size or image quality. If you plan to print many 8 by 10 prints, you should have at least a 3 megapixel camera, although I have gotten acceptable results printing 8 by 10s from a 2 megapixel original on a new photo-printer.
To review:
- PPI refers to image resolution
- DPI refers to printer resolution
- Inkjets need one-half to one-third the PPI of the printer's DPI setting
- Print size and print quality have an inverse relationship
- Print size and print quality are dependent on pixel resolution
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