For images that are being displayed on the screen, higher resolution does not equate to better clarity as it does with printed images. For screen images, resolution simply means size. If your resolution is higher, the image dimensions will be larger. Larger images may look clearer, but there is a trade-off in screen real estate. Although very small images are often seen as "fuzzy," the difference in quality as the images get larger is hardly noticeable.
What are pixels and ppi?
When working with image and screen resolution we talk about pixels. Pixels are simply blocks of color arranged in a grid. The grid makes up whatever image you are looking at. A screen and an image can both be measured in pixels. We use the term ppi or Pixels Per Inch. This specifies how many pixels represent a linear inch of an image or monitor screen.
When scanning a 3x5 inch photograph at 100 pixels per inch the resulting image becomes 300x500 pixels because each inch is represented by 100 separate pixels. Scanning an image at a higher resolution will result in a larger image. For example, scanning the same 3x5 inch photograph at 200 pixels per inch the resulting image becomes 600x1000 pixels because each inch of the picture is represented by 200 pixels.
A computer screen has its own resolution. The resolution is set in the Monitor (or Display) Control Panel by choosing screen dimensions. You can choose from dimensions such as 640x480 pixels or 800x600 pixels. Let's assume the screen is set to 800x600 pixels.
Since the screen can only display 800x600 pixels, a 300x500 image will take up almost half the width and almost all of the height. If the screen were set to 640x480 pixels, that same 300x500 image would be taller than the entire screen and you would have to scroll down to see the bottom of it.
So what resolution should I use when I scan?
By default, most scanners on campus, when set to Screen in the TWAIN software, will set the resolution to somewhere around 72 or 75. (Usually you will select the Screen setting from the Destination or Resolution menu in the TWAIN window.) Since increasing the resolution doesn't enhance the quality of your image, you are best to leave the resolution at the default and do your sizing using the scale feature in the TWAIN window. If that is all you are interested in knowing, stop reading. If you want to know the nitty-gritty of 72 ppi it is explained below.
Why is 72 ppi the standard? (otherwise known as the nitty-gritty)
To answer this question you need to know more about the screen resolution of your monitor. As was mentioned earlier, you can change the resolution of your screen by changing the dimensions in the Monitor (or Display) Control Panel. The resolution of your screen is determined by dividing the screen width in pixels by the visible width of the screen in inches.
For example: Set your screen to be 800x600 pixels in the Monitor (or Display) Control Panel. A 17 inch monitor has a viewable area that is about 12.5 inches wide. To find your screen resolution divide 800 by 12.5 and you will get a resolution of 64 ppi. Below are a few other calculations for screen resolution. Generally, monitors have a screen resolution between 60 and 100 ppi.
|Screen Dimensions from Monitor (or Display) Control Panel||Viewable Width of Monitor||Screen Resolution|
|800x600 pixels||12.5 inches (17" monitor)||800/12.5 = 64 ppi|
|1024x768 pixels||12.5 inches||1024/12.5 = 82 ppi|
|1024x768 pixels||14.6 inches (19" monitor)||1024/14.6 = 70 ppi|
|1280x1024 pixels||14.6 inches||1280/14.6 = 87 ppi|
So what does this mean? Almost nothing. Here is an example that will illustrate what part screen resolution has in influencing the display of an image.
Scan in a 3x5 inch picture at 72 ppi. Assume that your monitor also has a screen resolution of 72 ppi. This means that when you look at the picture on your monitor, it will show up at close to its original size. (I say close to because images are displayed slightly enlarged on monitors.) If your monitor had a screen resolution of 64 ppi, the image would show up slightly larger (as if it were a 3.4x5.6 image). If your monitor had a screen resolution of 82 ppi, the image would show up smaller (as if it were a 2.6x4.4 image).
Here's the math if you are interested...
My original image is 3x5 inches and I scan it in at 72 ppi
3x72 by 5x72 = 216x360 pixels
On a 72 ppi monitor 216/72 by 360/72 = 3x5 inches
On a 64 ppi monitor 216/64 by 360/72 is approx. 3.4x5.6 inches
On an 82 ppi monitor 216/82 by 306/82 is approx. 2.6x4.4 inches
This phenomenon can be seen when you change your settings in the Monitor (or Display) Control Panel. If you change your screen dimensions from 800x600 to 1024x832 (a higher resolution), suddenly all of your windows and icons are much smaller!
So, since you cannot determine the resolution of all of the screens that will be viewing your image, it is best to simply choose a number and 72 or 75 is just that. It is a "good enough" estimate of screen resolution.