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Image Resolution vs. Dimensions vs. Size: How the Numbers Stack Up

So when looking at an image file you have three basic numbers that you’ll want to pay attention to. The first is the DPI, which stands for “dots per inch” and is also the resolution of the image. To confuse matters, people make a very poor distinction between this number and the second, which is the number of pixels an image has in both height and width. These are the dimensions of the image, but many people call these numbers the resolution. (See, confusing!) The final number you need to understand is the image size, or how much storage space it takes up.

Now, let’s break this down, shall we? DPI is the actual number of ink dots in a one inch line. It is a physical printing term. The higher the DPI of an image, the clearer the image is when it is printed. In addition, higher DPI images will be displayed more clearly on high resolution displays, such as high end tablet and telephone screens. There is less of a difference in how an image appears from higher to lower DPI on standard computer monitors because of how such equipment is made. (This does not mean, however, that you should reduce the DPI of images to a lower level if they are only intended for web use.)istock_000020084611xsmall-1

The dimensions of an image are the number of actual pixels wide and high. Many people are used to a certain number of MP or megapixels in a specific image because of how digital cameras work. Changing the image dimensions reduces the number of MP in an image, and is a common way digital cameras can be tuned to take more pictures before running out of room. (But size is up next, remember!) An image with larger dimensions (and less DPI) can be printed out in larger format. So for example if a 5MP image that is 2338 x 3264 pixels is printed at 72 DPI you would get a 34 x 45.3 inch printout. That same image at 150 DPI would be 16.3 x 21.8 inches. 300 DPI is considered an appropriate print resolution for professional work, and that means that your image would now be printing at 8.2 x 10.9 inches. As you can see, the larger your images, the better blown up pictures you will be able to print out. (This is not the only thing that goes to quality, of course. A better camera sensor produces better images of course.)

Now, finally, we reach size. There are many factors which go into how much storage an image will take up. The larger the dimensions of an image the more space it will take up of course. But factors in the image itself have a great deal to do with this. As an example, using the same dimensions as above, 5MP images might range from as much as 4 megabytes to 0.5 megabytes. The first example would be a very complex image with a huge amount of color differentiation. The latter would be a very simple image with very little color deviation. This might be the difference between an image of a child behind a screen door on a sunny day as compared to a picture of light reflecting off of an object outdoors at night, with no flash.

So, now for these parting tips:

  • Do not reduce the dimensions of your master copies of your images!
  • When reducing the dimensions of your images, make sure you set them to a size appropriate for your medium, and no less.
  • When setting the DPI of your images for online use, always go with 300+ when possible.
  • When printing your images, go with the DPI that gets you the size of printed image you desire, while conforming to the printer’s specifications, but always use 300+ DPI for any high quality prints.
  • Never skimp on storage, and keep backups of your images. DVD discs can make a good backup because you can store them away from your PC.

You should have a good grasp of what the three numbers you need to keep track of are for your images now. Bearing them in mind can make it so you are always sharing the best quality images online, and getting only the highest quality printouts.

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