Need to put an image in a newsletter, or
brochure destined for the printer...
but don't know how big it should be?

You are not alone. This seems to be one of the most confusing issues in digital imaging. Here are some tips that will help you wade through the details with ease.

When you send something to be printed they convert your images into halftones. This means that they divide your continuous tone image into a lot of different size dots of color. This is done in order to be able to mix the colors of ink together to make all of the different colors in your image (this is also how they make the shades of gray in a black and white image).

The first thing that you need to determine, in order to know what resolution image to send to the printer, is what line screen your printer will use. The line screen is a measure of the size of dot that will make up your final printed image. Newspapers typically use a line screen of 80 or 100 lines per inch. This is expressed as 80lpi or 100lpi. Magazines usually are printed at 150lpi.

Your digital image is made up of another kind of dot called a pixel (picture element).  The size of a digital image is measured in pixels per inch (ppi).

To figure out how big your image needs to be take the lpi and multiply it by two. This will give you the number of pixels per inch that you will need for good image quality.

150lpi X 2 =300ppi.

This means that if you want to print an image 3x5 inches you will need an image that is 900ppi X 1500ppi.

3inches X 300lpi=900ppi and 5inches X 300lpi=1500ppi.

Lets take a look at this in reverse. If you have a camera that makes an image that is 640ppi X 480ppi you will be able to have it printed 2.1inches X 1.6inches maximum if you are using a 150 line screen.

640ppi / 300lpi=2.1inches and 480ppi / 300lpi=1.6inches

See if you can figure out how big you could print a 640ppi X 480ppi image with a 100 line screen.

Here is the answer