Resolution means how many pixels your image has in an inch.

Your monitor displays about 72 pixels (square dots) in an inch. Images that are saved with 72 ppi (pixels per inch) display on computer monitors in a 1:1 ratio. The dimensions of the image will equal the size of the display.
 
 

How Big is A Pixel?

Think about this for a moment. A pixel has no set size, until you define how many of them will be in one inch. When you set resolution to 72ppi, your image will be cut into these square grids, and each inch will have 72 x 72 tiny, one-color squares in each inch.

If your image has a resolution of 72 ppi, each pixel is 1/72 of an inch. The image displays 1:1. A 3x5 inch photo, saved at 72ppi will display on a computer monitor as 3 inches by 5 inches.

Ever Received a GIGANTIC photo in your email?

I know you've received photos in email that were so big you could only view the top left corner. You may have scrolled around the screen to see the rest of the picture, but I'll bet you wondered why your friend was sending you such a huge image. You probably also wondered, while the email was being sent to your mailbox, what the heck was tying up your computer for such a long time.

The picture was big like that because your friend scanned (or shot the image with his digital camera) at a higher resolution. If he used 150 ppi, you saw the image about 4 times its size. 150 ppi means each inch in the original photo got stretched to 2 inches when you displayed it on your monitor. The end result: a 6" x 10" image.

If he sent you an image at 300 ppi, that 3x5 photo is more than 12" x 20" -- much too large to display on a computer monitor. These are the ones you have to scroll around to see.

If he sent you something he scanned at a really high resolution, the file size was several megabytes, and probably knocked out your mail server to the point that you had to call your service provider and have the thing deleted at their end before you could receive any more email. You and your friend might have thought there was something wrong with the file.

There was. It was too big.

There is no reason at all for sending images or putting images on a web page at more than 72ppi. The extra pixels bloat the file size, even if you define the display size.

This means, you can tell your computer to display the giant file inside a 3"x5" area, but the file itself still contains all that data. The extra data is visually unimportant. Your monitor can't display it.

Extra pixels serve no purpose. They just make the file bigger, so it takes longer to download.

When might you want higher resolution?

You'll need to set a higher resolution in cases where:

1. You need to enlarge an original. Let's say you have a wallet sized snapshot, and you want to double the height and width. You would want to scan at 150ppi. The display will be bigger than the original, and will contain all the original image data.

2. You want to print out a photo-quality image on good paper. You'll need between 240 - 300 ppi to get a photo-quality print. Be sure to use photo quality paper, or the extra pixel data will only serve to slow printing time and make your paper soggy.

3. You want to correct an image in a program like Photoshop. You need enough data to work with, so scan at 240 or 300 ppi. After you've corrected the image, size it for the output. To print, leave it at 300 (or 240). If you want to display it on a monitor, resample it to 72 ppi, and define the size of the image, too. How you do this depends on the program you are using.

To resample in Photoshop:

1. Right-click the title bar o the image. Choose Resize Image.
2. Check the Resample box.
3. Set resolution, and set image size. Click OK.

The procedure will be similar in other programs.

Christine C. Frey