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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. 

Physical vs Memory Size

tiger image being printed

Physical Size

The physical size of an image refers to the dimensions in which it would print (ex: 8.5" x 11") or the pixels dimensions of an image displayed on the web (ex: 600 pixels x 800 pixels).

 

 

Memory Size

USB storage drive

The memory size is how much memory an image file takes up on a hard drive. For example, a JPG image saved on a computer may be 2 MB (megabytes), meaning that it will require 2MB of space on a drive to store that image. 

How do I resize?

Not all of our images are the exact size we need them to be, so it's important to understand how to properly resize an image and how resizing works. When an image is resized, its pixel information is changed. For example, an image is reduced in size, any unneeded pixel information will be discarded by the photo editor (Photoshop). When an image is enlarged, the photo editor must create and add new pixel information -- based on its best guesses -- to achieve a larger size which typically results in either a very pixelated or very soft and blurry looking image.

This is why it is much easier to downsize an image than it is to enlarge an image. If an image is needed for high-quality (publishing) or large format (poster) prints, be sure that it is captured using the highest resolution and quality possible because of the difficulty in enlarging.
 
 
Are you sure you need to resize or just change resolution? If you are unsure, check out the What is Resolution tab to learn about how resolution affects images. We recommend that you adjust your image's resolution first to the quality needed before resizing the image.

 

 

 

Scaling vs. Resizing

When working with raster images (pixel-based) it is important to understand that scaling an image in programs, such as Word, Powerpoint, InDesign, or Dreamweaver, does not actually resize the image, but rather stretches images larger or scales them smaller. When scaling, the resolution is not adjusted to best suit the new size, rather the pixels are stretched and can appear pixelated.

The most common side effect of scaling an image larger than its original dimensions is that the image may appear to be very fuzzy or pixelated. Scaling images smaller than the original dimensions does not affect quality as much, but can have other side effects. For example, if you upload a very large image to a website and scale it down to a smaller size, the website still must load the full size version of that image and could cause the web page to load more slowly.

 

Exceptions:

  • A very small amount of scaling can be OK, specifically when scaling down. However, we strongly recommended resizing images in photo editors, such as Photoshop or GIMP, to achieve maximum photo quality.
     
  • Vector graphics (such as clipart and charts in Word or Powerpoint) are not comprised of pixels and therefore can be stretched to any size without loss of quality.