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April 2011 |
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| Resolution, Pixels and DPI |
| What
Size is My Image ? The confusion begins when you view an image on a computer screen then try printing it to a printer. If you open an image file and display it on a computer screen (without zooming in or out on the image) then you will be looking at the image file at the resolution of the computer screen. If you then print the image, with no scaling applied, then it will print at the printer resolution. Take the example above, of 400 x 550 pixels. When viewed on a typical 72 dpi computer screen this image would appear to be 5.55 x 7.64 inches in size and would fill quite a portion of the screen.
The same image printed directly to a 720 dpi printer, with no scaling applied, would be only 0.55 x 0.764 inches is size and would occupy a tiny corner of the page.
From this you can see that the physical size (inches) of the image depends entirely upon the resolution (dpi) of the output device. For this reason it is often better to think of the image in terms of its size in pixels rather than its physical size in inches, as the pixel size does not change. Printing
Solutions If you want a printout that is 6 inches x 8.25 inches on a 720 dpi printer then you need to resize the image to be 4320 x 5940 pixels in size (i.e. 4320 = 6 x 720 and 5940 = 8.25 x 720). The drawback of this is that the image becomes very much larger in size and will be more difficult to work with. It will not be able see the entire image on screen at a zoom factor of 1 and the larger size will lead to longer processing times for certain operations. The second option is usually provided in the printer driver software. Options like "Scale to fit page" or "Print full page" are provided when printing. These options will take any image and resize the data sent to the printer in order to obtain the largest possible printed image. You should ensure that the "aspect ratio" is maintained when selecting these options; otherwise the image will become stretched or squashed on the paper. |
