Image Resolution

The concept of Dots Per Inch (DPI) is one of the most common misunderstood problems for computer users. The geeks explain DPI as well as the tools and tricks you can use to create clear pictures with your computer and printers. Additionally the geeks take your calls at (831) 476-2800 or toll-free at 1 (800) 655-5877 (from CA).

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h3. Links

University of Oregon has a useful page on digital
images… it simplifies the issues but addresses the needs of most
people.
About.com has a more detaild
explination of Desktop Publishing with regards to resolution. Which, if
you have the time, we recomend reading.

This DPI /Resolution thing is such a common problem. Or, I should say, I get asked all the time “What did I do wrong? This image looked good on my screen.” I think it is an extreamly difficult thing to understand compleatly, but here are some of my pointers:

Understand what you want to do with the image before you
get it in your computer. If you are printing the image with your home printer
read what the manufacture sugests for a good print. I think a 300 DPI image is
sufficent for a home printer.
Don’t scan at to high a resolution. You will increase
your file size and may never use the extra data, which you will still have to
store, manage and transport.
After you get your image into your computer and have
saved the file make a copy and work on the copy. That way you always have a good
version to work from. And you know what your “best quality image”
is.
Don’t trust that if it looks good on screen it will look
good when you print it. And if it looks bad it is mostlikely not your printer.
Instead it is probably what you did when you worked on the
file.
Don’t modify your image size and DPI unless you are
absolutly sure you know what you are doing. Making the image large with more
pixels is not productive. It increases your image filesize but not the data
information that the file holds.

Image Size Dialog in Photoshop
To see if you have an image file (TIFF, JPG) that will produce a
good quality print open it in Adobe Photoshop and, using the menue, go to Image
- Image Size. In this dialog you want the resolution to be 300 DPI (or possably better) and at the
width and height of the size you want to print (Like 5 inches
by 7 inches). Make sure the the “Resample Image” check box is not checked. The
“Constrain Proportions” should be checked and the width and height drop downs
should be on “Inches”. If the “Resample Image” box is checked the image will be
modify and you will loose data or try to create new data. So, make sure that
“Resample Image” is unchecked. Now change the numbers in the “Document Size”
portion of the dialog. And see what happens. As you increase the width and
height you decrease the DPI . They are connected. The dots that make up the image
are the information of that image. And the Dots Per Inch is a paramater you
assign to the image to make the information into a picture of a certain
size.
Another way to think about dots per inch is to think of it as “how
big do I want my dots to be?” The more dots per inch the smaller you want your
dots to be. If you want to have an image that looks really nice you need to have
about 300 dots to be eule to one inch long. If you have an image that is 72 DPI it means
that the dots have to be bigger to equal an inch. With the “Resample Image”
checkbox uncheaked change the DPI to 300. Now how big is the image (width and
height)? That is how big it can print and look good.

Image Size Dialog in Photoshop - too much information
If the width and hight are
larger then you need then you have enough information (in fact too much) to
print the image at the size you want.

Image Size Dialog in Photoshop - not enough information

If the width and hight are smaller then
you want you have a problem. You do not have enough image information to make a
good print at the size you want.

Optionally, change the width and height to be the size you want to print to. The DPI is now different. If the DPI is now lower then 300 then it will not

look that good when printed (200 DPI still looks good). If the DPI is larger then 300 you have more
information then you need and you can check the “Resample Image” and make the
paramaters what you want (300 DPI with the width and height the size for output)
- do a save as and send it off to the printer. Be carful though – when
resampling the image like this you are looksing information. Keep your
original.