What is DPI, PPI and Why Do They Matter?

To some extent, we're all photographers these days. With a camera on every phone and digital SLRs coming down in price, we've all got a trove of photos waiting to be shared. When it comes time to share online, print, or email our favorite images, many are unsure about how to set the image's resolution...

If you've found yourself in this spot, don't worry - dots per inch (shortened to DPI from here on out) is a concept that even confounds some professional graphic artists. Here's a primer DPI so you can stop worrying about technology and start sharing your photos.

Getting started
Digital photos are comprised of pixels, much like the individual boxes on a sheet of graph paper. DPI tells you how small those pixels will be when the image is printed. For example, "300 dots per inch" means that 300 pixels fit across each inch. If your photo is 600 pixels tall by 900 pixels wide, for example, it would come out at 2" x 3" inches if you were to print at 300 DPI. Keep in mind that most digital photos are several thousand pixels in either direction, but for the sake of simplicity, we'll use the more manageable 600 x 900 pixels.

Separating pixels from presentation
It's important to separate DPI from the raw pixel dimensions, and this is where even the pros slip up. DPI is not an indication of image quality or clarity. When you print that 600 x 900 pixel image at 300DPI, it'll likely look pretty sharp, because every inch is densely packed with pixels.

Now imagine printing that same image, with the same number of pixels, at a mere 30 DPI. As each inch would have only 30 pixels across, the density drops immensely and the image prints much larger: 20" by 30". What was once sharp now appears blurry, because each individual pixel is now ten times larger than before. By separating DPI from actual pixel count, we can understand that raising DPI doesn't magically improve a photo. DPI simply takes the same data (the original pixels) and alters how we'll view them.

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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.

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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 resolution

Whether 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)

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