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Guides to Photo and Document Preservation and Sharing for Families and Community Historical Societies

Printing Quality Photo and Document Digital Images

By R. Anderson - 2/2009

The single most important factor in producing quality prints from digital images is deciding on the pixel per inch (PPI) resolution to reflect in prints.  Rules of Thumb for pixels per inch are recommended in this article by consolidating thoughts from many sources.

Quality Printing Considerations

Affordable, easy to use, and very capable consumer printers now make it possible for non-professionals to produce their own high quality prints of digital photo and document images.

On the other hand, many local and online printing services offer reasonable pricing, consistent high quality, and a broad range of print sizes and image products.  With the easy ability to upload/transfer digital images to these printers, it often makes sound sense for families and local historical societies to turn to these professionals for many of their printing needs, particularly for photo realistic image enlargements.

  Regardless of how much printing you may have done professionally, it is useful to understand printing processes and terminology.  This will make you a better printer and/or consumer of printing services. 

 

Rules of Thumb for Photo Print Resolution

Cropping and aspect ratio considerations aside, digital image file pixel dimensions for height and width are spread over inches of paper in the printing process.  A given number of pixels spread over a 4" x 6" piece of paper will be reflected as more pixels per inch than the same number of pixels spread over a piece of paper that measures 8" x 10".  It is this density of pixels in the print that is the greatest impact on photo print quality.

Opinions vary as to what level of pixel density or resolution is needed to attain satisfactory print results.  It is generally agreed that the eye is not capable of discerning differences beyond 300 pixels per inch.  The following defined ranges are an attempt to consolidate the thoughts and recommendations of many.

300 PPI / Professional Photo Quality - The photo print resolution quality targeted by professionals.  There is broad agreement that one does not obtain a print quality advantage beyond 300 PPI.

240 to 300 PPI / High Photo Quality - Many agree that few people can discern a meaningful difference between 240 PPI and 300 PPI.  Therefore, for most non-professional image uses, this level of resolution should not compromise results.

200 to 240 PPI / High Quality - Among resolution recommendations of professional printers, this range is often described as very good quality.  (You may recall our recommendation to perform archival scans at 600 PPI.  This suggests that the 600 PPI scan can be tripled in print size and still maintain a print resolution of 200 PPI.)

150 to 200 PPI / Good Quality (Photo Realistic) - In this range, you may want to test prints at various sizes before committing resources to a major project.  Based on minimum resolution standards set by major online printers, you are still likely to be satisfied with print resolution in this range.

100 to 150 PPI / Acceptable Quality - Results in this range will vary based on the nature of the image, the size of the print, and the intended purpose of the print.  It is possible that results are still satisfactory.

 

Confusion Between Pixels Per Inch (PPI) and Dots Per Inch (DPI)

Frequent reference to dots per inch (DPI) often confuses the discussion of print resolution.  DPI is simply how many dots of ink a printer can lay down on an inch of paper.  It is generally agreed that when a printer is capable of DPI in the range of 600 for text to 1200 for photo realistic images, this is sufficient for most non-professional printing needs.  With modern printing technology, it is generally agreed that print quality is not improved noticeably beyond this, and today's moderate priced printers exceed these DPI levels.

In describing and recommending rules of thumb for printer image resolutions, our primary focus is placed on pixels available in a digital image file and how you choose to reflect them as pixels per inch (PPI) in the printed image. 

Printer Pixel Resolution Requirements for Families and Local Historical Societies

If you explore the websites of multiple printers, photographers, and other image related sites, you will see that printing resolution recommendations vary widely.  This is the case since prints are produced for different purposes, and resolution quality varies in importance to each purpose.  4" x 6" family trip photos serve one purpose, while a dramatic sunset with subtle color hue variations needs to meet a different printing standard in order to be enlarged and framed.

For a vast majority of family and community historical society projects, printed image resolutions well below 300 PPI professional quality is perfectly acceptable.  A quality representation of the image information we are seeking to preserve and share is our objective, and this can normally be accomplished with print resolutions in the range of 150+ PPI.

It is also useful to understand that as the size of the enlargement increases, the pixel resolution needed for acceptable viewing decreases.  This is the case since we normally view larger prints at a greater distance, and at this greater distance, our eyes are not as discerning.  Therefore, most online printers will accept orders for larger dimension prints (greater than 8" x 10") that will result in pixels per inch of less than 100 PPI.

The nature of the image and quality of the original print both have an impact on the results of these enlargements, since blemishes and quality shortcomings are magnified in larger prints.  Therefore, when you intend to print quantities of images at pixels dimensions of less than 150 PPI, or you are printing enlargements, you should test some images at your planned PPI to make sure that they are satisfactory for your particular project.

Online Printer Resolution Recommendations

The recommendations of large online printers are useful to consider.  These online printers have established their published resolution minimum standards based on their expertise, and these standards are supported by customer satisfaction experience.  Online printers can not afford a high rate of returns.  Therefore, their published minimum standards have been found satisfactory to a majority of their customers.  On the next page, we compare the minimum resolution standards of  Shutterfly.com  and   Snapfish.com by HP .

Shutterfly's recommendation for minimum pixel dimensions start with the assumption that most images will have the 1.33 aspect ratio associated with a compact digital camera.  The third column is the megapixels reflected in the pixel dimensions.  You can see that Shutterfly.com  suggests a minimum of roughly 2 megapixels for an 8 x 10 print and 3 plus megapixels for enlargements over 8 x 10.  The fourth column is simply the pixel dimensions divided by the print size inches to calculate how many uncropped pixels there are for each printed inch.  As the aspect ratio is different for each print size from the starting image's 1.33 aspect ratio, some default cropping will take place to "fit" the image to the specified print size. The last column indicates the resulting minimum default cropped PPI resolution recommended by  Shutterfly.com  for common print sizes.

 

Print Size Minimum
Recommended
Pixel Dimensions		 Megapixels Available
Pixels Per Inch		 Fit to Size
Default Cropped
PPI Resolution
4 x 6 1024 x 768 0.79 171 x 192 171
5 x 7 1152 x 864 0.99 165 x 173 165
 8 x 10 1600 x 1200 1.92 160 x 150 150
11 x 14 2048 x 1536 3.15 146 x 140 140
16 x 20 2048 x 1536 3.15 102 x 96 96
20 x 30 2048 x 1536 3.15 68 x 77 68

Snapfish's recommendation for minimum pixel dimensions start with the assumption that image pixel dimensions have already been cropped to the aspect ratio of the print size. You can also see that Snapfish.com by HP has taken a very different approach by suggesting that 90 pixels per inch is likely to be an acceptable level of pixel resolution across the range of print sizes.

Print Size Minimum
Recommended
Pixel Dimensions		 Megapixels Pixels Per Inch Pre-Cropped
PPI Resolution
4 x 6 540  x 360 0.19 90 x 90 90
5 x 7 630 x 450 0.28 90 x 90 90
 8 x 10 900 x 720 0.65 90 x 90 90
11 x 14 1260 x 990 1.25 90 x 90 90
16 x 20 1800 x 1440 2.59 90 x 90 90
20 x 30 2700 x 1800 4.86 90 x 90 90

Clearly, online printers prefer that pixel resolutions exceed these levels for better results.  The comparisons above illustrate that determining acceptable image quality is not an exact science.  Therefore, particularly when printing more expensive enlargements at lower PPI resolutions, it is a good idea to test a few images to make sure that they are satisfactory to your purposes.

Recall that in the scanning guide, it was suggested that you scan and photograph to specific needs when known.  For general purpose scanning, we suggested that you scan at 600 PPI.  In the table below, we see what this means to our printed PPI density, for various print sizes, when we scan a 4x6 original.

Print Size 4 x 6 Original
600 PPI Scan
Pixel Dimensions		 Megapixels Maximum Print
Pixels Per Inch
Available		 Fit to Size
Default Cropped
PPI Resolution
4 x 6 3600  x 2400 8.64 600 x 600 600
5 x 7 3600  x 2400 8.64 480 x 514 480
 8 x 10 3600  x 2400 8.64  300 x 360 300
11 x 14 3600  x 2400 8.64 218 x 257 218
16 x 20 3600  x 2400 8.64 150 x 120 120
20 x 30 3600  x 2400 8.64 120 x 120 120

You can see that the 600 PPI scan provides print resolution pixels across the range of print sizes that meets Shutterfly.com and Snapfish.com by HP minimums.  In fact, the number of pixels provide cushion for moderate cropping, and the likelihood that you may scan some images smaller than 4" x 6".

Dots Per Inch and Inkjet Printers

As described previously, pixels per inch (PPI) of image detail is a more important factor in print quality than dots per inch (DPI) of ink that a printer can lay down on a piece of paper. 

This is the case with up-to-date inkjet printers since typical printer DPI ratings now exceed the levels at which the eye can discern a meaningful difference. 

Since inkjet photo printers are the most common for non-professional general purpose use, it is useful to understand how they create photo realistic images with a limited number of colors in ink cartridges.

Recall that when a color or gray-scale digital image is captured, the digital file stores information about pixels in millions of color shades.  On the other hand, an inkjet printer must blend and transition a limited number of colors to attain the subtleties of a full color image.  To render the photo realistic image, a quality inkjet printer places set proportions of multiple dots of available colors of ink within a matrix to best represent the complexity of colors and hues of a given pixel.

For this reason, it is often useful for the inkjet printer to have a dots (of ink) per inch (DPI) capability in excess of the number of pixels per inch being reflected in the printed image.

Clearly, one does not need as many subtleties of color, or dots of varied colored inks, to print in black and white or other images of limited colors.

Tip / Hint: Assuming that you are using quality inks and paper, the following rules of thumb apply to desired dots  per inch of ink.
■    300   DPI  - Good quality text and routine documents
■    600   DPI  - Presentation quality printing
■  1200   DPI  - High quality photo printing
■  1200+ DPI  - Perhaps most useful to professionals

Additionally, printer manufacturers have built in adjustments for how their inks and printer technology works best with different types of paper.

With current moderate priced printers, you are normally asked to indicate the type of image you intend to print (photo, text, etc.), the type of paper you are printing on (plain, photo in several varieties, etc.), and the quality of the printing (highest photo quality to draft).

Based on these selections, the printer will use defaults for how many dots per inch of ink to print and how much ink to use.  With modern print technology, these defaults will typically serve you well and accurately reflect the pixel information that your digital image file provides.