|How To Scale An Image for Monitor Display or Printing|
While I believe it is very useful to understand the math and concepts presented on this page, many imaging programs and scanner and printer software control applications will do most of the scaling and sizing math transparently by allowing the user to just 'plug in' numbers for image, scan or print size. Understanding the math behind digital imaging will insure that you will have the knowledge necessary to produce images that are suitable for a wide variety of different output needs.
It is important to know what you intend to use your image for before you scan it or take the picture. As we mentioned previously, you will need higher pixel resolution files for printing than you will need for web or email attachment use. In digital image creation and workflow pixel resolution (dimensions in pixels) is the first and most important decision. 'What pixel resolution do we need to print to 8X10 inches at 300 ppi output resolution?' -- ' What is the scanning dpi necessary to get 2400X3000 pixels from my 35mm film?' -- 'What pixel resolution does it take to fill up half my monitor display when it is running 1024X768 resolution?' We will adjust the DPI of the scanner or the resolution setting of the digital camera or resize the image in our editing program to scale the digital image to the pixel resolution that is required for a specified print size or for display on a computer monitor.
Pixel resolution and assigned PPI (print or output resolution) are independent of each other but the ratio between the pixel dimensions of your digital image and the PPI selected for printing is what determines the printed size of your photograph. Or, in the case of scanning, the selected scanning DPI in relation to the size of your original scanned media is what determines the pixel resolution (dimensions) of your scanned image.
It is important to understand that DPI is a user assigned number for scaling media to a required pixel resolution during scanning. PPI is used to scale existing digital images to a required size for printing (output resolution). After you have scanned your media or taken a digital photo at your selected pixel resolution you can then change the assigned PPI number to scale the image's pixels to print at any size on paper. Just remember that 240-360ppi is the threshold for photo quality. Changing the assigned PPI after a digital image is created does not change it's pixel resolution. Changing the assigned PPI of an existing digital image only changes print size that would be created when printing that digital image.
For printing purposes each pixel in your digital image is going to be converted into ONE DOT of color information on your print. This matrix of 'dots' when viewed at normal viewing distances give the viewer the illusion of a continuous tone photograph. As mentioned above, printed output generated at 240-360 PPI is considered by most to be a safe threshold for seeing digital prints as a continuous tone photograph.
You don't need to be concerned about your home printer's native output resolution. If your printer is capable of 1440X2800 dpi output the printer will do the necessary math and interpolation behind the scenes to make the 8X10 at 300 ppi image you have scaled in your image printing program. The 240-360 ppi threshold we have given is an 'equivalent' standard. In other words, if you have enough pixels to print to required size at 240 ppi or higher output resolution, you will get photo quality prints from any of the current batch of home printers. Some of the higher end commercial printing machines like Fuji's Frontier system, LightJet, and Chromira do a little better with 300 ppi and above.
There are 4 related variables involved that can be arranged in simple math equations that will allow to you solve all of your digital image resolution and scaling requirements. The four variables are as follows: PIXEL RESOLUTION - DPI/PPI - ORIGINAL MEDIA SIZE - PRINTED OUTPUT SIZE.
Some Formulas For Scaling DPI/PPI For Scanning Or Printing
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