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Pixels Resolution and Image Size |
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Resolution and image size one of the most
confusing topics in digital photography.
What resolution should I use for printing or
my website?
Lets start with some background information
about pixel's
Digital images are made up of pixels, small
squares of digital information with a color
value.
A pixel's size will depend on the resolution
of the image. With two files of the same
dimensions, a file with a resolution of 150
pixels per inch will have larger pixels than
a file of 300 pixels per inch.
PPI stands for Pixels Per Inch and
represents the number of pixels per linear
inch on a photo print when a digital image's
pixels are scaled onto paper.
Digital photos should be referred to as
pixels per inch (ppi) or pixels per
centimeter (ppcm) – not dots per inch (dpi).
The file is made of pixels and the image on
the screen is made of pixels. In printing,
dots of ink are laid down on paper and dots
per inch (dpi) should be used.
Therefore ppi is for digital images and
screen images, and dpi for prints. |
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The number of pixels per inch is associated
when the dimensions of the image creates the
image resolution. The more data that is
recorded by a digital camera, the larger the
file size. 300 ppi in itself is not a higher
resolution than 150 ppi; it means nothing
until the image dimensions are included.
For example
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An image of 150 ppi and size of 8" x 10" is
1200 pixels x 1500 pixels
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An image of 300 ppi and size of 8" x 10" is
2400 pixels x 3000 pixels
The 8x10 @ 300ppi is going to be a higher
resolution than 8x10 @ 150ppi and larger
file size |
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However the 8x10 @ 150ppi will will be the
same resolution as a 4x5 @ 300ppi, they both
have a file size of 5.15 megabytes.
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Another example would be an image with
dimensions of 1200 pixels by 1800 pixels
with an assigned print resolution of 300 PPI
would print to 4 by 6 inches. Height and
width dimensions of the image in pixels
divided by assigned PPI. 1200 pixels divided
by 300 = 4 and 1800 divided by 300 = 6. |
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The most important is the actual pixel
dimensions. The second is print resolution.
Print resolution scales the existing pixel
dimensions to a requested print or document
size.
You need higher pixel resolution for
printing than you will for websites or email
attachments. This is because most photo
printing is done between 240 and 360 dpi.
The human eye sees photo prints with printer
output resolutions equivalent to 240 ppi and
above as continuous tone image even though
the image is made up of slightly overlapping
dots. The higher the output resolution (more
dots per inch) the more subtle color shade
are in the printed photograph. Higher output
resolutions also produce sharper details in
the image.
Computer monitor displays are between 72-96
dpi depending on the monitor. |
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Megapixels
Megapixels describe the maximum number of
pixel available in a digital camera.
Megapixel resolution is the product of the
camera CCD's length and width pixel
resolution. A camera with a 3648 by 2736
pixel CCD has 'approximately' 10 megapixel
maximum image resolution (3648 times 2736 =
9,980,928 pixels/1,000,000 = 9.980928
megapixels). |
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Printing
To determin the necessary resolution for
printing to a required print size. Multiply
the length and the width of your intended
print size (in inches) by 300 (printing at
the equivalent of 300 PPI is universally
accepted as generating photo quality output)
This will give you the length and width in
pixels that you need to print to your
required size with photo quality. Some
printers will work with 240 ppi. 300 is a
safe minimum when you don't know the actual
requirements of your printer.
PPI is used to scale digital images to a
required size for printing. You can change
the assigned PPI number to scale the image's
pixels to print at any size on paper.
Changing the PPI after a digital image is
created does not change the number of pixel.
Changing the PPI of a digital image only
changes print size that would be created
when printing that digital image.
Notice the file size, pixels and document
size in the example below. |
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When printing 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. |
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