Scanner resolution

by Andy Rush

Here is a practical demonstration of how scanner resolution settings affect the picture size on the computer screen. Resolution is measured in dots per inch or dpi. Scanners will be able to digitize an image at many different resolution settings, and their manufacturers will usually advertise (hype) their highest resolution. 600 dpi! 1200 dpi! 2400 dpi! 9600 dpi! STOP THE MADNESS!!! In most cases, scanning at 100 dpi is sufficient.

For this demonstration, we took an ordinary photograph with dimensions of 5 inches wide by 3 1/2 inches high. It was scanned at 100, 300, and 600 dpi. The image scanned at 100 dpi is shown first. Because the resolution settings affect the physical size of the image on a web page, we won’t show the full 300 and 600 dpi images. They would be overwhelmingly large. You’ll have to just imagine how they would look full size. We do, however, represent these images at their higher resolutions by showing you the “manification effect”. First, take a look at the 100 dpi image:

100 DPI

This is the original photo converted from a .tif format. The dimensions were approximately 5 x 3 1/2″. This image was scanned at 100 dpi . The resulting digital image is 489×349 pixels . Note the direct relationship to its size in pixels: For the width: 100 dots per inch, times almost 5 inches = 489 dots or 489 pixels For the height: 100 dots per inch, times 3 and one half inches = 349 dots or 349 pixels The file size of the resulting .tif was about .5 MB. The compressed jpeg image that you see is 22KB.

Now let’s look at a representation of a 300 dpi image. Since we cannot show the full image here, we “crop” the image to the same dimensions as the first image above. Note the resulting “magnification” effect.

300 DPI

The original photo was scanned this time at 300 dpi and cropped to the dimension of the original photo (489×349). The full size of this image un-cropped would be 1467×1047 pixels. If we displayed this image on the web page at full size, you would have to scroll to see the whole image. For the width: 300 dots per inch, times almost 5 inches = 1467 dots or 1467 pixels For the height: 300 dots per inch, times 3 and one half inches = 1047 dots or 1047 pixels This image represents a 3 times magnification of the 100 dpi image. The file size of the resulting .tif was about 4.5MB.

Next is the image scanned at 600 dpi and cropped to the same size as the 100 dpi image.

600 DPI

The original photo was scanned at 600 dpi and cropped to the dimension of the top photo (489×349). The full size of this image would be 2934×2094 pixels.  Imagine trying to display the full image on a web page. For the width: 600 dots per inch, times almost 5 inches = 2934 dots or 2934 pixels For the height: 600 dots per inch, times 3 and one half inches = 2094 dots or 2094 pixels This image represents a 6 times magnification of the 100 dpi image. The file size of the resulting .tif was about 17MB.

Let’s look at it another way . . .

300 DPI

In the above image each square represents an area of 489×349 pixels, or the size of the original 100 dpi image at the top of this page. For a 300 dpi image there are 9 squares measuring 489×349.

600 DPI

In the above image each square represents an area of 489×349 pixels, or the size of the original 100 dpi image at the top of this page. For the 600 dpi image there are 36 squares(!) measuring 489×349.Now it should be clear why you don’t need high resolution settings to scan images for web pages.