Photo Resolution: A Quick Tutorial

Today, the idea of dots per inch (dpi) is often confused with pixels per inch (ppi). So much so that they’re often used interchangeably. But if you need to send an image to print, understanding how dpi/ppi affects your resolution and print quality is a must. To help us out on the matter, David Bergman shares this two-minute tip:

Though it may seem pretty technical, image resolution is really a simple concept. Digital photos are images made up of pixels. Each pixel contains information that, when resolved with other pixels, creates your final image. The more pixels per inch, the more information in your photo and, as a result, a higher image quality. And while a high resolution isn’t particularly necessary for how most of use images on the web, it is necessary for a high quality printed image. And that’s where ppi (and in the past, dpi) really matters.

Back in the days when a digital image was printed using a 1 to 1 ratio (1 pixel = 1 printer dot), the idea of dots per inch actually mattered. But since modern printers use a blended dot and completely remap your image when converting it to print, the idea of dots per inch has gone by the wayside. These days, if you want to print an image, all you need to be concerned about is pixels per inch (though many folks still use the terms interchangeably).

How Photo Resolution Works

Let’s say someone asks for a 300 ppi image that will be used for an 8″ x 10″ print, and you have an 8 Mp image with the pixel dimensions of 3264 x 2448. If you divide those dimensions by your print size, you’ll get 326.4 ppi for the longer dimension and 306 ppi for the shorter. Since it’s the smaller number that matters (it’s fine to be above the minimum ppi, but not below it), with a bit of cropping your image will print at 306 ppi on 8″ x 10″ paper. Since it’s always okay exceed the minimum ppi, you’ve met the 300 ppi request.

Divide those dimensions by your print size and you’ll get 326.4 ppi for the long dimension and 306 ppi for the short dimension. They are different because the aspect ratio (length to width) is different (a common problem), so the smaller number applies, your digital photo will print to 306 ppi on 8″ x 10″ paper (with a bit of cropping). It would meet the request for 300 ppi at 8″ x 10″ (more pixels are okay).

Understanding Resolution

Of course, most image-editing software will do the math for you, but you have to be careful when using it. It’s pretty easy to unintentionally re-size your image, leading to all sorts of headaches. In fact, in most programs this setting is just a conversion calculator, letting you know what the printed size will be at a specific ppi and vice versa. The resolution itself, though, lies exclusively in the amount of pixels per inch and the print size intended.

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