What is Resolution?

What Resolution?

Does that “Print Size” button under the Photoshop Magnifying Glass tool give you night sweats?
What resolution do you save your image at for web viewing? 72ppi? 72dpi? 96? Does it matter? What is the difference between DPI and PPI anyway?

In this post I’ll explain to you the difference between PPI and DPI and what someone means when they tell you they need a file “at 300PPI” I’ll also help you to get the “Print Size” button to ACTUALLY show you your print at print size! Grab your monitor and a ruler/tape measure, and hold on to your hat.

An In-Depth Doctoral Level Treatise Comparing, Contrasting, and Elucidating the Distinct Differences Between DPI and PPI.

For all intents and purposes, there isn’t any….
PPI stands for “Pixels Per Inch,” DPI is “Dots Per Inch.”  PPI is generally used for imaging devices (cameras, scanners) and monitors, while DPI refers to the resolution of printers.  An image file itself has NO INHERINT RESOLUTION, it has a number of pixels for width, and a number of pixels for height.  The PPI “resolution” of the image is meaningless.  It is the pixels that matter.  And the DPI of the printer. 

If someone asks you for a 300PPI (or DPI) file, ask them what the final print size is, because of that pesky "Per Inch" part, you don't know how many pixels to give them if you don't know how many inches they need to print.  I'm also always being asked things like: "I need a 5MB file."
Again, that means nothing.  Pixels are king, the only thing i need to know, is how many pixels do you need. If you need a 5x7 at 300PPI, you've told me how many pixels you need, 1500x2100, if you just tell me 300PPI, i have no way of knowing what size file you need.  5MB is indeed a file size, but it has nothing to do with the resolution of the image.

We use wet process photo printers, they expose the photo paper with RGB lasers for continuous tone printing, there are no actual “dots” that make up the image (unlike inkjet/dry lab printers) but for optimum quality, these photo printers expect files to have enough pixels to cover 300DPI at print size.
To make an 8x10 print, the file should be no smaller than 8*300=2400pixels by 10*300=3000 pixels, 2400x3000.  You can go into Photoshop, change the resolution setting to 72PPI…. As long as the pixel dimensions stay 2400x3000, your 8x10 will be at 300DPI… Changing the Resolution setting, while keeping the pixel dimensions the same, doesn’t resize the image, it just reshuffles the rulers, and tells you how big the file will print if printed at the resolution you chose.

So if a client calls you up and asks if you give them a file a 240DPI, tell them “Absolutely!” Then ask what size the image will be printed.  If it’s a yearbook photo, say it’s going to be printed 2 ¼ x 2 ¾, at for 240 DPI you need to provide them with a file that is 2.25*240 X 2.75*240 or 540pixels by 660pixels.

What is the standard for monitor resolution? Or the web? 72PPI? 96PPI?

There isn’t one…. 72PPI was a standard for the ORIGINAL Macintosh’s 9” screen. Why 72PPI? Because the Macintosh’s screen was meant to work together well with Apple’s ImageWriter printers which printed at a resolution of 144DPI, double the screen resolution.  Apple kept its larger screens at 72PPI so their users could (as long as they were still using ImageWriters) have an accurate representation of what they were going to get out of their printers.

Where did 96DPI on Windows machines come from?

Yes, that’s a screen cap from Windows XP, yes, it reads “DPI,” even Bill Gates finds PPI/DPI confusing….
Why 96? Because it’s 1/3 more than 72…. Makes perfect sense right?

Comfortable reading distance tends to be just under arms’ length, Microsoft figured most people were further than arms’ length from their monitors, about 1/3 further away.  A monitor that is further away, needs to display text larger for it to be comfortable to read. About 1/3 bigger.  If you have a 72PPI monitor set up to mimic the look of printed text, that text will be tougher to read, so we’ll design our programs to display text (and icons and stuff) at 96PPI (excuse me, I mean DPI)

72/3=24, 72+24=96

Now, monitors have just gotten larger and larger with higher pixel dimensions, that’s why XP had that DPI adjustment box, if the icons and text on your monitor are too small for your aging eyes, just up the resolution from 96 to 120 or so… in Windows 7 they decided to ditch the whole PPI/DPI thing and make it easier for everyone…. “Make it bigger, or biggerer”

Your monitor doesn’t care what PPI (or DPI) your image is set at, neither does the web, all they care about are pixels.  And the number of pixels in your image is determined by how big you want the image to look on your screen, or what size it needs to be to fit your web design.  I save web images at 96PPI out of habit, but if I have a 600pixel hole in the web page to fill, it doesn’t matter if the file resolution is set to 72, 96, 300 or 5000PPI….. as long as it’s 600pixels wide, it’ll fit the hole…


Photoshop “Print Size” why doesn’t it show me the image…. At print size?

Because Photoshop doesn’t know how big your monitor is, or what its resolution is.
But don’t worry, I’ll explain to you how to tell Photoshop this info.

“Print Size” was accurate when your Mac Classic was hooked up to your Apple Imagewriter, it hasn’t been accurate since then unfortunately.  I’m typing this and screen capping these images on a 24” monitor.  If I open up a 5x7 (@300PPI) image on my screen and click the “Print Size” view and hold a ruler up to my monitor, the image actually measures 5” by 7”

Open Photoshop right now, create a new document 5x7@300 (1500pixels by 2100pixels), click “Print Size” and grab a ruler.

How did you make out?

Probably not very well, because Photoshop assumes your monitor resolution is 72PPI. 

The first step is to make sure your monitor is running at its “Native Resolution,” windows calls this the “Recommended Size” for this 14” Eizo monitor, the Native Res is 1920x1200.

Write down whatever the WIDTH of your screen is in pixels, 1920 in my case.  Now measure the width of the actual screen, not the diagonal (how monitors are marketed) and don’t include the frame around it, just the screen itself, round to the nearest decimal.  My 24” monitor is 20 7/16” wide, 7/16 = 0.4375 call it 20.4375”

Now we’re looking for our monitor’s resolution, its PIXELS PER INCH, we know how many pixels it has, 1920 and we know how many inches wide it is, 20.4375, divide and we get our resolution!

1920/20.4375=93.9449 call it 94PPI, a lot higher than 72!

In CS6 the adjustment is under “Edit,” “Preferences,” “Units & Rulers”

I enter my calculated PPI under “Screen Resolution,” “Print Resolution” should be set to 300 (or whatever DPI your printer requires)

An inch is an inch!