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Understanding Resolution, pixels and DPI
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Ever wondered what it means that your
monitor has a display resolution
of 800 x 600 pixels? Or that your flatbed scanner supports
a maximum resolution of 1200 ppi? As will be explained
below, these concepts are related.
On a piece of paper, the size of an image is measured in inches. In a digital
device, like a monitor or a camera, the size of an image is measured in pixels. A pixel is
the smallest basic unit that composes a digital image. A lot of small pixels
put together make up an image.
Pixels cannot be measured in inches. The size of
one pixel depends on the quantity of pixels that compose an image of a fixed
size.
A 6" x 4" photograph scanned at 150 "pixels per inch",
or ppi, will generate 900 pixels on the horizontal side and 600 pixels
on the vertical side. The same picture scanned at 300 ppi
generates 1800 x 1200 pixels. The more pixels an image has,
the more detail that can
be seen with the naked eye, and thus, the better resolution of the image.
Take a look at the following image. It was scanned both at 150 ppi and 600 ppi.
Then we zoomed into the house on the right.
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See what happens when zooming into the images? Look
closely at the roof's borders on each picture. Higher
resolution allows us to zoom in as deep without losing
as much detail, and also to
get a good definition when printing blown-up images.
Understanding
about resolution is also very important when determining
printing size. Regular ink-jet printers print at 300
dots per inch, or dpi, which is a similar concept to "ppi".
The more pixels an image (of a fixed
size) has, the more it can be enlarged when printing
without losing detail. So, if you want to scan and then
enlarge or zoom in for printing, you should go for higher
resolutions when scanning.
Resolution is also important
because pixel quantity determines computer file sizes.
Each pixel is represented by 3 bytes. So, a 5 megapixel
image will result in a 15 million bytes
(15 MB) uncompressed TIFF file. A 6 x 4" photograph
scanned at 300 ppi will create a 2.2 MB uncompressed
TIFF file. Other file formats such
as JPEG allow for increasing levels of compression, and
thus, decreasing file sizes.
For a lengthier discussion
on resolution and pixels (in PDF format), click
here.
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