DPI, what is it and what are the myths.

Posted by in Printing and Display

DPI stands for dots per inch, and applies to the resolution on the printed page.  You may also have heard of LPI (lines per inch) and PPI (Pixels Per Inch) those are also ways of describing resolution.  Most photographic printers print great at 240 or 300 DPI this is refereed to as their resolution. You may have also heard of the “magical” 72 DPI that is supposedly you screen resolution, well guess what, its not.  Computer screens are capable of displaying different resolutions and for example the 24” iMac is 94DPI at its native resolution of 1920×1200.  I would have to set the resolution of my screen to 1467×918 to achieve the magical 72 DPI, and guess what, the images would look horrible.   So what does DPI have to do with an image? Well its quite easy it tells Photoshop how to display the ruler on the screen, and tells the printer what the “preferred” size to print the image.

Why am I writing this, well there are two reasons:

  1. I wanted you to know when to talk DPI and when to talk pixel size.
  2. I needed a article for the blog.

Now lets talk about publishing to the web. This is where most people think that DPI plays an important part in image quality.   Web pages do not work that way, they work on the number of pixels in your image. So guess what, a 2000×2000 pixel image at 72 DPI will look just as great as a 2000×2000 pixel image at 300dpi on your monitor.  Why? Because your screen will display the image the way the web page tells it to display, and web pages ignore DPI.  So when someone tells you to send them a 72 DPI images, what they should really say is, “please send me a low res version of that wonderful image, 800 pixels wide should do the trick.”

When I export for the web I usually stick to 800 pixels wide, this will give a large enough image so it looks great on a large screen, but small enough not to print well. When I send a image in for the local photo contest, where they ask for a 72 DPI image, I send a landscape image at 1024 pixels wide and a portrait image at 768. Why these weird sizes you ask, well its because the projector resolution is 1024×768 and I would prefer to have Photoshop resize the image for me rather than their viewing software.

Am I worried about people stealing my images on the internets? Yes and no.  First of all, do not post anything on the web you do not want stolen. How can you show your great stuff on the internets if you do not post it? That’s the challenge.  You need to make that leap of faith if you want to “share” your photos on the web.  You could always place a watermark on the images, it’s better to have a ugly watermark than not show the image, right?

I hope clears up more than it confuses.

Bill Debevc