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DPI/PPI: Image Resolution And Print Size.

PPI (pixels per inch) is a digital (electronic display) term.  DPI (dots per inch) is a physical (printed
out with ink on paper or another surface) term.  When dealing with taking a digital image (taken with
a digital camera) and printing it out, all you really need to know is this:
The image's actual size in pixels:  I have a 14 Megapixel camera; a picture taken with this
camera is 4288 pixels wide by 3216 pixels high (4288 x 3216px).
Desired printed resolution to get a decent-looking photographic image:  for normal print uses
figure this as 300 dpi.  
To figure out how big my printed photo will be if printed at 300 dpi, take the dimensions in
pixels and divide by 300:  
     4288 / 300 = 14.29
     3216 / 300 = 10.72
At 300 dpi, the picture's printed size will be 14.29" wide by 10.72" high.
In other words, the only numbers you need to worry about when you want to print out an image are
the image's height and width in pixels.  If you want the picture to look good in print, it needs to be at
least 300 dpi, so divide the height by 300, and the width by 300, and that tells you the largest size
that you can print the image at  and have it look good.  
In the example above, at 300 dpi the image would be 14.29 x 10.72", which is too large to print on
a regular printer.  Does that mean you can't print it?  Not at all!  Go ahead and print it at 11 x 8 1/2".  
The resolution will be even higher than 300 dpi, which means the picture will be even better quality.
Another example:  If you have a digital image that is 600 x 300px, at 300 dpi the printed image will
be only 2 x 1" (600/300 = 2; 300/300 = 1).  You could print the image at 4 x 2", which would give
you a resolution of 150 dpi (600/150 = 4; 300/150 = 2) - the picture won't be quite as crisp, but will
still be fair-looking.  
The larger you print the image, the uglier it will get!  If you printed out that same 600 x 300px image
at 72 dpi, the resulting picture would be about 8.3 x 4.16" - but it would look lousy.
How does this work in practice?  Figure out the maximum size you can print a given image (at
300dpi).  When you go to print out the image (whether you print it directly from the photo editing
software, or paste the image into a document and then print it), tell the computer the finished size
(in inches) that you want the printed picture to be.  
(My apologies to my fellow Canadians, by the way - I used inches for many years before the
conversion to metric, and I still think in inches when it comes to images - as do the people who
coined the terms "dots per INCH" and "pixels per INCH", obviously!  Just FYI, an inch is
approximately 2.5 cm - I'll let you do the math!)
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