It's all about context Another factor is viewing distance. Just think of the eye chart at your doctor's office. If you're a bit nearsighted, the tiny letters at the bottom are illegible specks, while the letters at the top are easily discerned. In actuality, each tiny letter may be half an inch tall, but the distance makes them seem microscopic. Now consider our 600 by 900 pixel image. When we printed it at 30 DPI, the giant pixels made it look blurry. Were we to look at it across the doctor's office long hallway, however, it may look just as sharp as the 300 DPI print did in our hands. This illustrates how DPI is more about context than quality.
Pixels Per Inch You'll notice I've been talking about DPI in relation to printing only. This is because while printers can produce a variety of DPI settings, a computer display's resolution is fixed - its pixel density is part of the physical hardware, and cannot be altered. When talking about displays instead of print, most use the term PPI, or "pixels per inch." If you intend to put your 600 x 900 pixel image online, switching the resolution to 30, 300, or 3000 PPI is completely arbitrary, because the computer display can't change its density. As modern desktop displays usually have a PPI in the low 100s, the 600 x 900 pixel image will appear around 6" by 9" (mobile displays may be much higher). Of course, your web browser could display the image smaller if need be, but it will do so by averaging and eliminating pixels, not squeezing them to be physically smaller. This is why it's always important to keep your end goal in mind when working with images. In summary:
• An image is defined by its pixel dimensions - # pixels tall by # pixels wide • DPI/PPI determines the scale and pixel density at which image will be displayed • What appears blurry from close up may look fine at a distance, so consider how an image will be seen • Printers can produce a range of DPIs, while displays have fixed resolutionWhether you're a blogger dealing with an upload limit or are just trying to print a photo to hang on the wall, understanding DPI/PPI can go a long way. I hope these tips help you feel more in control of your images and how you share them with the world! Images: Gregory Han, Mark Grambau (dog photo by Adrienne Breaux)