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Getting Started Scanning...
Part 3: Printing Digital Photos & Scans

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...)


 More of this Feature
• Introduction
• Part 1: DPI for Email & Web
• Part 2: Saving Your Scans
• Part 3: Printing Digital Photos & Scans
 
 Join the Discussion
"A handy rule of thumb is: for desktop printers (inkjets, lasers) use a PPI equivalent to around one-third the printer resolution."
- EFX1
 
"Always keep in mind what you are going to be using the scan for and plan for it. You will get a better scan than if you scan it at 4x6 and then say print 8x10."
- COMPUTERCREA
 Related Resources
• Glossary: Resolution
• Glossary: TWAIN
• Digital Imaging & Scanning
• Scanning Tips
• Scanning Software
• Photo Printing Software
• Increasing Image Resolution
• Prep Your Photos for the Web
 
 From Other Guides
• The DPI Dilemma
• Scanning Line Art
• Scanning Basics
• Resolution Inch By Inch
• Top Picks: Glossy Paper
• Top Picks: Matte Paper
 
 Elsewhere on the Web
• A Few Scanning Tips
• DPI for Printing
• Printing Your Scans (Paper)
 

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.

Bitmap images are made up of individual pixels.

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:
300 x 300 or 320 x 320 dpi
high quality:
600 x 600 or 720 x 720 dpi, 1440 x 720
photo quality:
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:

 
Printer Quality
Draft/Economy
Standard
High/Photo
Print @
300 - 320 dpi
600 - 720 dpi
1200 - 2880+ dpi
Scan @
150 ppi
150-240 ppi
240-360 ppi
Printed Size
Actual Pixel Dimensions (Average)
2" x 3"
300 x 450 pixels
400 x 600 pixels
600 x 900 pixels
4" x 6"
600 x 900 pixels
800 x 1200 pixels
1200 x 1800 pixels
5" x 7"
750 x 1050 pixels
1000 x 1400 pixels
1500 x 2100 pixels
8" x 10"
1200 x 1500 pixels
1600 x 2000 pixels
2400 x 3000 pixels

Generally, printing photographic images requires more pixels per inch. For photo printing, you will get better results using the higher end of the scale. For scanned illustrations, cartoons, or paintings, you can use a lower PPI and not see a noticeable change in the printed quality.

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

--> Back to the Intro: Getting Started Scanning

Further Reading...

Increasing Image Resolution
How to increase image resolution with minimal loss in image quality. Get bigger photo prints at higher quality. Several methods and software options are explained.

DPI for Printing
Find out here what DPI to scan at when the image is to be printed on your inkjet printer.

Printing your Scans
Describes the different types of paper for printing scans on an inkjet printer along with visual examples showing the results of each type of paper.

Resolution: DPI, SPI, LPI, & PPI
Graphic Design Guide Judy Litt helps you understand the different types of resolution and what resolution to use.

More About Scanning

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