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 and printing it out, all you really need to know is this:
The image's actual size in pixels: With a 14 Megapixel camera, a picture taken at maximum size will be 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. (You can get
away with 200dpi, but the photo won't be as clear).
To figure out how big the 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.
NOTE: check your printer and your image software; for instance, I had a printer that automatically printed JPG files at 72ppi regardless of
how the resolution was set in the software!
(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!)