Sample Scans - Understanding Image Quality vs. File Size

This reference page was originally created for optimizing files for download over dial-up modem. While hi-speed internet connections are becoming common place, this page is still relevant, especially when considering a scanned document storage solution.

Sample Scans



Sample Document - demonstrates color and smaller print
Color and gray images saved JPEG, compressed in MS Photo Editor at 90%, (best quality is 100%).
Black and White images saved GIF.

To determine download time in seconds:
at 32kb/s, divide file size by 4 (1000KB = 250sec)
at 48kb/s, divide file size by 6 (1000KB = 167sec)
at 120kb/s, divide file size by 15 (1000KB = 67sec)
at 500kb/s, divide file size by 62 (1000KB = 16sec)
at 1000kb/s, divide file size by 125 (1000KB = 8sec)

Computer Image Concepts


With the advent of digital cameras and electronic document storage, understanding image files is becoming important.

Balancing image quality to file size can be confusing.

Common Questions:
- Why do I need more mega-pixels if I can not see them?
- Why does my camera's memory stick fill up so quickly?

Obviously, the more image detail, the larger the file.
Large files are usually undesirable:
  - takes a long time to send through the Internet
  - consumes more disk space (hard drive, cd-rom, USB drive, etc.)
  - slower to load, and consumes more memory

The primary advantage to a high quality large image is greater detail.

In the sample document below, a standard 8.5" x 11.0" page was scanned at various dots per inch (DPI). The document was also scanned in color, gray, and black and white.

Dots in an image are also referred to as pixels. Each pixel defines a dot of color, put all of the dots together and the image is defined. The ideal number of pixels to store an image would provide just enough detail so that the image does not appear 'grainy'.

The size of an image should be relative to how the image will be looked at.

To view an image on your computer monitor, the image does not need to be larger than your screen area. Most computer users have their monitor set to 1024 x 768 pixels or 1280 x 1024 pixels (the number of pixels across the screen by the number of pixels top to bottom). To check your setting, right click anywhere on your desktop area (not on an icon), then select 'Properties'. The window titled 'Display Properties' will appear, select the tab labeled 'Settings' and note the value under the slider labeled 'Desktop Area'.

To view an image full screen with more pixels than what your screen is capable of displaying does not improve the image. Therefore an image taken with a digital camera with a setting of 1600 x 1200 will look exactly the same as an image taken at 800 x 600 pixels, when viewed at 800 x 600. But the file will be about 4 times larger.

To view an image printed out, you will probably be asked what size to print out the image as. I have found that 150 pixels per inch will be the greatest level of detail you will be able to see on a printed image, any more DPI will not provide a better printout on all but the highest quality printers.

A standard piece of paper is 8.5" x 11.0", subtract for the usually required 1/2" margin and you have about 7.5" x 10.0" to print on. At 150 dpi, this would be an image size of 1500 x 1125 pixels. A digital camera set to 1600 x 1200 will print on a standard piece of paper at about 160 dpi, which will usually be quite satisfactory. Take a calculator, and multiply 1600 x 1200 you get 1,920,000 pixels. Divide this number by a million and you get 1.92 Mega pixels. A digital camera with more than 1.92 Mega pixels is great to have as an option, but rarely will this additional resolution be useful.

Large file sizes can be dificult to manage as detailed above. For example, a 1600 x 1200 pixel image will be about 850KiloBytes (KB) in size. Since a floppy disk can hold 1440KB, only one image would fit on a floppy disk. And a standard modem speed of 50kb/s would take about 170 seconds (almost 3 minutes) to download through a web page or through the email. Archiving images to a CD-Burner would allow for about 765 images on a standard 650MB CD-Recordable disc.

By comparison, an 800 x 600 image is usually about 220KB in size. You could fit 6 images on a floppy disc, 2955 on a CD-Recordable, and take 44 seconds to download by 50k modem.

For internet applications, I set my Sony Cybershot digital camera to 640 x 480 (.3 Mega pixels). Usual image size is 60KB, which computes to 24 images on a floppy, 12 seconds to download by 50k modem. Since I usually have more than one image on a web page (or in an email), I usually create a 'thumbnail' version that is 320 x 240 pixels or 15KB in size. A 15KB image takes only 3 seconds to download by 50k modem, so I can have three images on a web page download in less than 10 seconds. Many computer users cannot connect by modem at greater than 28.8kb/s, which would double the above example download times.

A good example of this practice can be seen at the website: Nancy's K9 Home Page. Smaller images (320 x 240) are provided in the web pages, any image can be clicked on by the visitor to display a larger image (640 x 480). This allows for a web page to be loaded fairly quickly with adequate images, but a means to view greater detail images should the visitor choose to do so. A 640 x 480 image printed at 150 dots per inch would be 4.26" x 3.2", comparable to a conventional photo.

Electronic document archiving follows the same concept.

Compressing images can also be an effective means of reducing file size. There are several image standards in use that use different methods to reduce file size while minimizing loss of image quality. One of the most common formats is called 'J-PEG', the file name will usually end in '.jpg'. Most image programs are able to cut a J-PEG file size in half with minor visible difference.





Sample Document, at various DPI and Color format


 

Color

Gray

B&W

75


637x825

Thumbnail 200%
Full Page 100% 177KB
Full Page 200%
Standard Page 133%

Thumbnail 200%
Full Page 100% 170KB
Full Page 200%
Standard Page 133%

Thumbnail 200%
Full Page 100% 28KB
Full Page 200%
Standard Page 133%

100


850x1100

Thumbnail 200%
Full Page 100% 285KB
Full Page 200%
Standard Page 100%

Thumbnail 200%
Full Page 100% 278KB
Full Page 200%
Standard Page 100%

Thumbnail 200%
Full Page 100% 47KB
Full Page 200%
Standard Page 100%

150


1275x1650

Thumbnail 200%
Full Page 100% 543KB
Full Page 200%
Standard Page 67%

Thumbnail 200%
Full Page 100% 498KB
Full Page 200%
Standard Page 67%

Thumbnail 200%
Full Page 100% 90KB
Full Page 200%
Standard Page 67%

200


1700x2200

Thumbnail 200%
Full Page 100% 775KB
Full Page 200%
Standard Page 50%

Thumbnail 200%
Full Page 100% 702KB
Full Page 200%
Standard Page 50%

Thumbnail 200%
Full Page 100% 138KB
Full Page 200%
Standard Page 50%

300


2550x3300

Thumbnail 200%
Full Page 100% 1316KB
Full Page 200%
Standard Page 33%

Thumbnail 200%
Full Page 100% 1155KB
Full Page 200%
Standard Page 33%

Thumbnail 200%
Full Page 100% 257KB
Full Page 200%
Standard Page 33%

Grayscale 100 DPI, at various quality factors


Full size image, no compression: 553KB
 
Quality Factor = 90
Full Page 278KB

Quality Factor = 80
Full Page 213KB

Quality Factor = 70
Full Page 181KB

 
Quality Factor = 60
Full Page 159KB

Quality Factor = 50
Full Page 144KB

Quality Factor = 40
Full Page 129KB

 
Quality Factor = 30
Full Page 112KB

Quality Factor = 20
Full Page 91KB

Quality Factor = 10
Full Page 63KB

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