Understanding The Correct Image Resolution

It seems since the Internet was invented, few fully understand resolution, or the proper resolution when it comes to web browsers, social media, and printing. There are many myths, facts and even theories, but one thing for sure is that the pixel resolution convolution comes from misinformation, marketing hype and history.

Let’s learn the proper terminology in resolution first. When it comes to digital image resolution there are three types of resolution we must deal with, pixels per inch (PPI), dots per inch (DPI) and lines per inch (LPI). Since digital cameras are everywhere, and less scanning, we’ll skip LPI, which is how traditional scanners scan and commercial offset printers print.

PPI is the most important unit of measure when it comes to cameras and screens (monitors). A camera captures in PPI and the monitors display in PPI, plus most image editing programs work in PPI, like Adobe Photoshop and Adobe Lightroom. The latter are the industry standards when it comes to photo editing and editing of photos—there is a difference.

Open up your preferences panel in Adobe Photoshop, then go to Units & Rulers; there you will see many things, but notice your Print Resolution and Screen Resolution settings. The defaults are 72 PPI for screen resolution and 300 DPI for print resolution; no need to change them, I just wanted you to see these are the defaults, which also adds to the pixel resolution convolution.

Pixel Resolution Preferences Photo

This photo showing where to find your pixel resolution preferences is saved at 72 PPI.

One reason the screen resolution is set to 72 PPI is because of fonts. Postscript fonts are based on 72 “points” per inch and 12 points per “pica,” thus a pica is 1/72 of a foot or 1/6th of an inch. Notice the 72 in 1/72. In publications and e-pubs, images normally accompany fonts, and fonts are harder to recreate on a screen than most images, hence why image editing programs default to 72 for Points/Pica resolution when it comes to fonts.

The other reason is the history. The first Mac computers from the 1980’s displayed 72 PPI so they could match Apple’s “ImageWriter” printer outputs of 144 DPI. You could literally measure your fonts, layout and design, etc., on the Mac 9-inch screen with a ruler or pica pole, and it was fairly exact to the ImageWriter output. One inch on the screen equaled one inch on the paper.

Pixel Resolution Preferences Photo

This photo showing pixel resolution preferences is saved at 300 PPI.

Windows’ monitors back then displayed 96 PPI and you had to multiply by 133% when you measured the screen (96 / 72 = 1.33) to match your screen to your printer. Ironically today, with new technology and flat panel screens, most monitors display somewhere between 109 to over 400 PPI and most people don’t put pica poles or rulers up to their screen to measure print output.

Throw in smart phones to the pixel resolution convolution along with browsers and what a mess. Since Apple started this with 72 PPI, and for good reason, let’s look at their iPhones. According to Apple, the iPhone 6 Plus is 1920-by-1080-pixel resolution at 401 PPI, whereas the iPhone 6, 5S and 5 all display at 326 PPI and iPads vary from 326 PPI and below.

Confused? Well take your monitor’s native resolution width and divide it by the actual image area display width (not diagonally across, but left to right) and you’ll probably find a resolution from 109 pixels per inch and up, not 72 PPI or 96 PPI—but in reality, it doesn’t matter what PPI resolution you save your photos at when it comes to monitors, smart phones or browsers.

You can save your photos at any resolution you’d like because what matters for screen (display) resolution are the actual total pixels in the image, not the PPI. An image 400 by 700 pixels saved at 72 PPI is the same file size, and screen size at 180, 300, 500, etc., pixels per inch. Resolution is only important for printing. The Golden Rule is, “Image Resolution Affects Print Size, It Does Not Affect Screen Size.”

Pixel Resolution Dimensions at 72 DPI

Take note of the pixel dimensions and the pixel resolution of this photo at 72 PPI.

Unless you’re keeping one for your old computer collection, the 72 and 96 PPI screens are nonexistent! Image size on your screen depends on the actual pixel dimensions of your image and the your display resolution, both are PPI. If you set your monitor’s screen resolution to its native display resolution your image will be displayed pixel-for-pixel exact size.

Image Dimension Size Pixel Resolution

Here again, notice how this image at 300 PPI shows the same image dimension size in pixels! There is no difference! The only thing that has change is the size of the print from 48 by 64 inches to 11 by 15 inches. The display size in your browser or screen doesn’t change, it’s still 3456 pixels by 4608 pixels in dimension.

The same goes for Facebook as you’re viewing images on Facebook via a screen. Where people have issues with Facebook is the proper size as Facebook compression algorithms suck! So far no one knows the magic formula for making Facebook uploaded images look their best, but the most common agreed upon solution is to size your photos at either 720, 960, or 2048 pixels on their longest side, then use your compression settings to make ensure the file size is 100KB or below. The resolution PPI setting doesn’t matter, but if it makes you feel good, leave it at 72 PPI. (See: JPEG, Save For Web or Save As, Don’t Be Fooled)

Now let’s go to DPI for printing because this is where it matters! First, let’s understand the history. The 300 DPI default comes from early heavy marketing hype of the old 300 DPI laser printers. Back then there were no Epson inkjets and when the Epson’s came out, they printed everything from 720 DPI and eventually evolved to 2880 DPI. The funny thing though, is most people, thanks to the pixel resolution convolution, send prints to their Epson inkjet printers at 300 DPI.

Ask yourself, how many times does 300 DPI go into 720 DPI, 1440 DPI, and 2880 DPI? Where do the remainder pixels go? I’m not even going answer this because there are many theories then the next thing you know we’ll get into raster image processing or RIPs; but it’s best to say, do your own testing with your printer and you be the judge. Find what works for you and stick to it. (Note: For e-books or e-pubs, check with your publishing platform for their resolution and pixel dimension requirements which vary, especially for Retina displays; but these requirements are not for viewing photos on social media, this is for actual publications and images displayed through the e-format on various book reading devices.)

Bottom line, don’t confuse PPI with DPI and remember, resolution really matters when it comes to printing because it determines the size of the final print. Screen resolution is about the amount of pixels, not PPI, in an image for your screen size not the amount of dots on paper—the more dots you lay down, generally the better the image quality of the print.

With that I close as always, please don’t forget the men and women who serve to protect our nation and our freedoms. God Bless them, their families and their friends, Rolando.

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