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There is often confusion
over the way the resolution of digital images is expressed. One of the simplest
and most useful ways of expressing the resolution of a digital image is in
pixels. A pixel is one of the many coloured dots that make up a digital image. As
an example, you may have an image that is 1500 x 1000 pixels. Our example would
have 1,500 dots in the horizontal direction and 1,000 dots in the vertical
direction.
An image that has a large amount of pixels also has a higher resolution.
High-resolution images will generally use up more disk space, occupy more
memory and take up more space on your computer's monitor when displayed in
full size. Having high-resolution images becomes necessary, however, if
you want to print your image and have it look like a photo, rather than a
large collection of little coloured squares!
Another very common way of referring to the resolution of digital images
is by dpi, which stands for dots per inch. You may also see ppi,
which stands for pixels per inch and has a nearly identical meaning. You
can estimate the number of dots per inch in an image by dividing the
images dimensions in pixels, by the dimensions of the image in inches. The
important thing to keep in mind about dpi is that it is always relative to
something.
Resolution(pixels)
= dpi x image size (inches)
If someone tells you they have an image that is 300 dpi, the actual image
resolution could be just about any size depending on the height and width
of the original scanned image. As an example, an 8x10 print scanned into a
flatbed scanner at 300 dpi, would be 2400 x 3000 pixels in size. (8 x
300=2400; 10 x 300=3000). But a 35mm slide scanned at 300 dpi would only
be 405 x 270 pixels, which would not be suitable for print use unless the
printed image was very small.
Converting DPI Measurements to Pixels and Vice Versa
Resolution (pixels) / Image Size (inches) = DPI
DPI x Image Size (Inches) = Resolution (pixels)
Beyond
resolution, an important expression of the quality of an image is its
color depth. Color depth is expressed in 'bits.' For example, most high
quality image files are saved in 24-bit color. Bits are an expression of
binary data and the number of bits in an image is an expression of the
number of colors that are possible in each pixel of the image. In a 24-bit
image, each pixel in the image has the potential of 16,777,216 different
colors. (Each bit represents a doubling in quantity, so an 8-bit image
would have 256 colors, a 9-bit image would have 512 colors, etc.)
From
looking at the examples below, you will notice that images with lower
color depths may appear to be "grainy" and the color transitions
are not as smooth. Needless to say, you will want to have your display
settings configured to show the maximum amount of colors available from
your video card, otherwise you may not be viewing the true potential of
images you see on your monitor.
So
why use images with lower color depth if they don't look as good? Simple.
They take up a lot less space. You will often see 8-bit images on the web,
as they can be downloaded a lot faster than larger-bit images. A process
called 'dithering' is often used when converting an image to a lower color
depth. Dithering involves inserting a pattern of different colored pixels
into an image to create the illusion of a single color. This process is
designed to make color transitions appear smoother in images with low
color depths. Dithering can also increase file sizes, but not as much as
an image with a higher color depth will, so it's often worth the tradeoff.
Dithering is also used when printing images, as most digital images
contain a lot more colors than printers have colors of ink.
Image
Resolution vs. Print Quality
To
get an idea of the potential print quality of a digital image before you
print it, you will want to estimate the number of dots per inch relative
to the size at which the image will be printed. To do this, divide the
number of pixels in each direction of the image by the number of inches in
the print dimensions. For example, if you have a 1500x1000 pixel image and
print it at 6 inches by 4 inches, it will work out to be 250 dpi when you
print the image.
For
most purposes, a file of this resolution will work out great. At 250 dpi,
you should get 'photo quality' results. It may be worth noting that for
professional publishing purposes, the standard is to have images that are
a minimum of 300 dpi. You'll find you can get by with less than that and
still have pleasing results for average everyday use, however.
On the low end, you probably never want to print an image at much less
than 100 dpi. Below that you will start to see degradation. If you shoot
for 200 dpi or higher, that should be suitable for most general
applications.
It may be helpful to work backwards sometimes. This way, you can estimate
how big you can print your digital image based on its pixel size. As an
example, if you have a 900x600 pixel image, you could divide that by 300
dpi and figure that you’ll need to print it at 3 inches by 2 inches or
less to get professional quality publishing results. Similarly, if you
divide it by 100 dpi, you can figure that you definitely don’t want to
print it any larger than 9 inches by 6 inches, otherwise the result will
not be satisfactory.
Other
Printing Factors to Consider
Now
comes the confusing part. You may be thinking, "I just bought a new
inkjet printer that prints at 1200 dpi. What good does that do me if I
only need a 300 dpi image to get professional results?" This is where
the color depth of your images comes into play. (See Resolution 101 for
more information on color depth.) If you have a 24-bit color image, there
are over 16 million shades of colors that can be used in each pixel of the
image as you see it on your monitor. Unfortunately, your printer only has
a handful of different ink colors.
In order for your printer to get your image onto paper, your image needs
to be converted from "RGB" color to "CMYK." RGB stands
for Red Green Blue. Colors you see on your monitor are projected
optically, and are made up of different combinations of Red Green and
Blue. But printed images are made up of four different ink colors: Cyan,
Magenta, Yellow and Black (CMYK).
The
conversion from RGB to CMYK is a rather complicated process. In order to
simulate the millions of different colors in your original image and
smooth color transitions, your printer puts many dots very close to one
another in order to simulate different color shades. This process is known
as 'dithering' or 'half toning.'
Thankfully, the software that comes with your printer will probably take
care of everything for you. It's helpful to understand, however, that some
colors possible in one format cannot be duplicated in another and
therefore substitutions must be used. This is one reason you will often
find that an image on your monitor almost always looks different once you
print it out on paper. The software that came with your printer may
provide provisions for color synchronization. This may help you to get
your desired result, but for the average user’s applications this is not
usually necessary.
Sending/Receiving Pictures
Have you ever received or tried to
send an email that had a picture attached? Has it taken you 2 hours
to receive the picture 
and when it finally finished downloading,
and you tried to view it, the computer protested?
There are many
different ways of saving a picture (called 'formats') onto your hard
drive. Some of the more popular formats are jpg, gif,
pcx, tif, bmp. If you are planning on sending a picture via
email to somebody, you will need to take special care to save the image
in either a gif or jpg format.
Why use only gif or jpg formats?
There are two reasons. The first and probably most
important is that almost anybody on the Internet can view these two
formats. That is not true of the others. It would be very easy to send a
file in a format that the person on the other end could not view...and it
happens frequently. By sticking with a gif or jpg, you'll be
assured that the person on the other end can see your picture.
The second reason to use one of these two formats is size. Generally,
these two formats take less space on your hard drive, which means it will
take less time for you to send the picture and less time for the person on
the other end to receive the picture. So, the next time you scan
some pictures to send, remember to save them as either a gif or jpg.
Generally you will use the jpg format for photographs and the gif
format for pictures that have fewer colors, like drawings or comic
strips.
You may need to consult the manual for your graphic/image
program to learn how to save images in the different formats.
Sometimes, to convert a picture from any format to a gif or
jpeg, you select that picture in its own programme, Copy it, and then open
another imaging programme, like Microsoft Image Composer, Adobe, Photoshop, and
others....then Paste your pic there...SAVE AS...name.jpeg...in
most modern imaging software, the gif/jpeg extensions are part of the drop down
menu options...Then it is simply a matter of identifying your file, when you are
inserting it in your email
There are two ways of inserting pictures in
your Email letter:-
In Internet Explorer, for example, click
Insert/Picture/Browse to find your pic.. If your Picture command is not
active, make sure Format/Rich Text (HTML) is activated. If the recipient can
still not see your picture, click Tools/Options/Send/HTML/Send pics with
messages is selected.
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