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Understanding digital images and resolution

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Saved by Jayashree Balaji
on February 4, 2011 at 3:25:40 pm


This unit covers the basics of digital images, image capture devices and adapters, features of digital images such as bit depth, pixels, measures of image resolution and how they vary based on their purpose whether display or printing.


At the end of  this unit lesson you will be able to:


  • Differentiate between screen and print resolution
  • Differentiate and appropriately use image display and printing measures PPI and DPI 
  • Explain the effects of resolution on file size




An important aspect of any project aimed at developing electronic resources is the production and management of image files. Images, video clips, flash images and animations can be used as fundamental Learning Objects or as part of more complex Learning Objects. There are various image- capture devices and image-capture capabilities are changing rapidly. It is important to appreciate the issues surrounding image quality and file formats. These will vary from collection to collection, and will change constantly due to rapid technological developments and emerging standards. In this unit we will look at the features of digital images which will impact file size and resolution. These are important considerations in the generation and dissemination of learning objects that we create. 


Pixels, bit depth and graphic file size


Digital images are composed of very small picture elements—called pixels—lined up in rows and columns. The size, color and brightness of each pixel determine how the image appears on the computer monitor or printer. Because pixels are the smallest picture elements, the pixel size determines how much detail will appear in the image: the smaller the pixels, the more pixels can appear in a given area, which results in higher resolution.




The pixel size of a file is determined when a digital image is created by the image-capture device and its software. The resolution of computer monitors is defined by the number of pixels found in one row multiplied by the number of pixels found in one column (e.g. 1024 X 780 pixels). Each pixel includes information that must be stored in the computer’s memory or other device. Therefore, the greater the resolution, the more the number of pixels, and the larger the image file has to be. When the resolution is very low, it results in loss of image information and inaccurate representation of the original image. On the other hand, too high a resolution increases the file size and may not necessarily provide valuable information to the viewer. Large files are also more difficult to maneuver because they take longer to be transmitted or load onto a web page.


Bit depth represents the number of values available to define each pixel.  Eight-bit images can display 256 possible values for each pixel, 16–bit images can display 65,536 possible values and 24–bit images 16,777,216 possible values. Because bit depth is information associated with each pixel, the greater the bit depth, the larger the image file. The human eye has its limitations, so excessive bit depth wastes memory space in digital imaging.




Resolution refers to the dots of ink or electronic pixels that make up a picture when it is printed on paper or displayed on-screen.





Pixel resolution image from: http://en.wikipedia.org/wiki/Image_resolution


Standard high resolution monitors display 1,024 X 780 pixels. The spatial resolution of computer monitors is generally 72 to 100 lines per inch, corresponding to pixel resolutions of 72 to 100 pixels per inch (PPI).  


Measures of resolution


PPI (pixels per inch) is the number of pixels displayed in an image. A digital image is composed of samples that the screen displays in pixels. The PPI is the display resolution, and not the image resolution. 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 is a function of the monitor. A pixel or "picture element" is a specific x,y coordinate (dot) on our screen. A high-resolution setting displays more pixels per inch.  


DPI (dots per inch) is a measure of the resolution of a printer referring to how many dots of ink or toner a printer can place within an inch (or centimeter). DPI is printer resolution. DPI is irrelevant on the web because images display on-screen in pixels (PPI).



A 10 × 10-pixel image on a computer display requires many more than 10 × 10 printer dots to be accurately reproduced on paper. Source: http://en.wikipedia.org/wiki/Dots_per_inch


LPI (lines per inch)  is an important measurement related to the way printers reproduce photographic images. The LPI is dependent on the output device and the type of paper.


SPI (samples per inch) is scanner and digital image resolution. A digital image is composed of samples, basically information about how to display that image. To scan an image the scanner takes a sampling of portions of the image. The more samples it takes per inch, the closer the scan is to the original image. The SPI is its scanning resolution or the amount and type of information stored for that image. The higher the resolution, the higher the SPI. This term is not commonly used.


Print and screen resolution


A printer’s dot is over 300 to 1000% smaller that a pixel on a monitor. This is why images made for the web are not suitable for printing. A high resolution image can always be downsized/compressed for the web. But a magnified image never looks good when printed. Although we can print images with a higher resolution than that displayed on most monitors, the screen resolution represents the “useful” resolution. Computer monitors typically display images at 72 or 96 PPI. Much less sampling (SPI) is needed for on-screen display and the information in images beyond 96 SPI is wasted, will not enhance the display, and unnecessarily increases the file size and download time.




Try this activity on image size


If you were to

  1. Scan a 1" x 1" photograph at 300 SPI
  2. Using any photo editing tool, compress the 300 SPI image to 100 SPI at 1" x 1" 
  3. Now increase the 300 SPI image size to 3" x 3" (300px X 300px)

What is the difference you see with file sizes on the screen after the change in resolution?



We have addressed four parameters that are important to understand the difference between how a digital image displays on a computer monitor and how it displays when printed on paper. The impact of tinkering with these features has also been addressed, this would help the learner effect changes to file size and resolution to create images suitable for web display or print display.

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