Image Size
and Resolution
A D V E R T I S E M E N T
Contrary to what you might
think, ppi is not the same as dpi. Almost everyone these days has a inkjet
printer so let's think about your inkjet printer for a moment. How many colors
of ink are in your inkjet printer? Most of them have at least four and possibly
six ink colors. When your printer lays down ink, it lays it down in overlapping
dots of these four or six colors. Images are made up of
pixels
and Photoshop defines resolution in pixels per inch. In a typical RGB image, a
single pixel represents one color out of the millions of colors that can be
reproduced on-screen.
So how can a printer with
only six colors of ink reproduce millions of colors? Well, certainly not by
printing a single dot for each pixel in your image! In reality, for each pixel
in an image, your printer may lay down several dots of overlapping color that
mix together to make up the color of that single pixel. All this is said to help
you understand why you would use an image resolution setting of 150 ppi in
Photoshop in order to output an image with a printed resolution of 300 dpi. It's
simple; your printer needs to use more than one dot to represent the color of a
single pixel.
To clear up another
confusing matter, resolution (ppi or dpi) has no bearing at all for images that
will be viewed on screen. The only thing that means anything for screen display
is the pixel dimensions; that is, the number of pixels that make up the width
and the number of pixels that make up the height of the image.
If you've been told that
you need to use 72 or 96 ppi for the Web, it's only because that is
approximately what a typical monitor can display. But, because we have the
ability to adjust the pixel dimensions of our monitor, and because monitors come
in varying sizes, an image with the same pixel dimensions is not always going to
appear the same size on screen on every system and with every monitor. However,
an image that is 100 by 200 pixels will always be 100 x 200 pixels, unless
you
resample that image. And that brings us right back to the image size dialog
box...
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