Understanding Resolution

This is one area that gets very technical, and alot people just skip over it.

Here is a layman's attempt to give clarify the topic.

Resolution of an image is measured in a variety of ways.
One way is DPI. That is Dots Per Inch. This measure is really only important when it comes to printing. And it doesn't factor in when we are just aiming to put our picture on a web page or in a PowerPoint presentation.

A better more broadly applicable measure is to look at the total Pixel Dimensions.
For example a 720x720 pixel image on a computer screen is going to
appear as approximately 10 inches. This is because most computer screens display at 72 dots per inch. Actually more modern high defintion screens can now display graphics at much higher densities.

But depending on how many dots per inch you define it as,

This 720x720 pixel image can be printed out in different sizes with varying levels of quality.

DPI Quality Printed Image Size
360 This image will appear to have more fine detail 2" x 2"
100 This image will be bigger but might reveal flaws 7.2 x 7.2

So to have a 7.2" image with the same detail as the 2" one you would need an image that is 2592 pixels wide and high (360 x 7.2) . Get it?

This is a scan of print out of 108 KB image / 400 x 400 pixels

Jgp printed with pixelation

It looked like this on the web


In computer graphics there are often two competing objectives that make things harder. On the one hand you want high quality. On the other hand you want high speed and portability. These two objectives often conflict.

So here when we took a photo that was made small for the web and try to send to a high resolution printer its defects are revealed. It would have looked ok if we tried to print it as a one inch by one inch !

When your final output is a printed hard copy the goal is have as much data as possible. This means high quality, high resolution and large file size. Very often if you try to go with too low a resolution you will get blotchy or rough looking prints. This is what happens when you take a image from the web and try to get a nice looking print out of it.

When your final output is the computer screen either via the web or your own hard drive, the goal is to have fast load times. Here you want small files sizes that don't compromise the quality too much so as to make the image unreadable.

 

Here is a scan of high resolution image printed at 300DPI. The image is originally about 3000 x 2200 pixels and is 6.8 MB compressed as TIFF. You can see its pretty sharp compared to the first image above but its not the type of image that a web developer would want to use for sure.