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Library Research Guides

All About Images

This comprehensive guide focuses on many common image questions in regards to image resolutions, resizing images, file types, vector and raster images, scanning, saving and more.

PPI and DPI

What are PPI & DPI?

These two acronyms are often used interchangeably although they do have different meanings.

  • PPI (Pixels Per Inch) refers display resolution, or, how many individual pixels are displayed in one inch of a digital image.
  • DPI (Dots Per Inch) refers to printer resolution, or, the number of dots of ink on a printed image.

This guide will use only PPI (Pixels Per Inch) to describe resolution. 

How to Change Image Resolution Using GIMP

Image editing programs like GIMP (GNU Image Manipulation Program) can give you resolution information and allow you to change the resolution of an image. One would change the resolution of an image particularly for printing purposes, because the quality of the print is dependent on the resolution of the image.

 

1. With GIMP open, go to File > Open and select an image


Tip:  You may right-click and save the tiger image below to use it as a practice image.

 

 

2. Go to Image > Print Size


 

3. A Set Image Print Resolution dialog box will appear like the one pictured below.

If Print Size Width and Heights are not shown as inches, select the dropdown beside Height and select "in".

 

4. In the X and Y Resolution fields, type in your desired resolution.

You will notice that when you type a value into the Resolution field, the values of the document's width and height also change. This is because GIMP is changing only the resolution of the image and not adding any additional pixels (which is what happens when an image is resized). To change resolution we are NOT changing the number of pixels in the photo, but changing only how many of those pixels will be displayed per inch.

 

5. Click OK to accept the changes.

Congratulations! You have successfully changed the resolution of an image! 

In this example, we had an image with a 300ppi resolution. I wanted to print this image in a professional publication and the image needed to be at least 600ppi. 

Remember that the number of pixels in the image have stayed the same because we did not add or subtract any pixels to the image, only determined how many of those pixels to display per inch.

However, note that the Width and Height decreased by half when the Resolutions doubled. This means that in order to print at 600ppi and retain full-quality, I can print this image only as large as 5" x 3.33".



What happens if we decrease our resolution?

As you may have guessed, our Width and Height doubled when we halved our Resolutions. Now my image will print larger, but the quality will be much lower.


 

What does it all mean?

It's a game of give and take!

We notice that the pixel dimensions never change. We started with an image at 3000 x 2000 pixels (px) and ended with the dimensions still being 3000 x 2000px. This is important to remember, because when we change resolution we are changing only how many pixels will be displayed per inch of the image, not how many pixels make up the image. Since we are not adding or subtracting any information (pixels) from our image, our image has to always balance out to its original 3000 x 2000 px. If we increase resolution then we must decrease from somewhere! Since the pixel dimensions cannot change, the only other place to decrease is our document size (width and height of image).

Here's the math to make it more clear: Sample Image is 3000 x 2000 px

600ppi: 3000 pixels / 600 pixels per inch = 5 inches

                  2000 pixels / 600 pixels per inch = 3.33 inches
 

300ppi: 3000 pixels / 300 pixels per inch = 10 inches

                 2000 pixels / 300 pixels per inch = 6.667 inches


150ppi: 3000 pixels / 150 pixels per inch = 20 inches

                 2000 pixels / 150 pixels per inch = 13.33 inches

 

72ppi:  3000 pixels / 72 pixels per inch = 41.67 inches

                2000 pixels / 72 pixels per inch = 27.78 inches

 

How does resolution affect printing?

In this example, our image has been sent to print from a laser printer on standard 8.5 x 11 sheets of paper.

72ppi: The document size is so large that it cannot fit onto an 8.5 x 11 sheet and is clipped. The print quality is extremely low, leaving the image looking very blurry or "soft". 

150ppi: The document size is still too large for the 8.5 x 11 sheet and the quality is mediocre and makes the image look acceptable, but isn't very desirable.

300ppi: The image almost fills up the entire sheet of paper and the print quality is very good with crisp, sharp details.

600ppi: The image is substantially smaller than the other files, but the quality is extremely high.

Which one to choose? The 72ppi and 150ppi images are too low of quality to produce a high-quality print, so they're out. The 300ppi and 600ppi images both looked very crisp, but the 600ppi image was way too small. For this example, the 300ppi image would work best because of it's larger print size and high quality.

The key to printing images is to find the best resolution that will produce both the size and quality you need. 

 

Screen/Web Resolution

Images for Web

Web images work differently than printed images. With printed images, we must pay close attention to resolution to ensure we get a high-quality print.

For web images, we must focus on the pixel dimensions. Look at the two images below - one is 300ppi and the other is 72ppi.

Pixel Dimensions 300 x 200; resolution 300ppi
(Pixel Dimensions 300 x 200; Resolution 300ppi)

 

Pixel Dimensions 300 x 200; resolution 72ppi
(Pixel Dimensions 300 x 200; Resolution 72ppi)

Both of these images display at the exact same size even though their resolution varies. This is because the pixel dimensions are what really matter when working with web images. Notice that the pixel dimensions of each image are the same and therefore both images will display at the same size even though they have differing resolutions.