Elsewhere I’ve written about resolution, primarily as it applies to sensor design. In this article I’m aiming at a less lofty goal … clarifying the difference between Pixels Per Inch (PPI) and Dots Per Inch (DPI). There are lots of explanations out there, but perhaps one more won’t hurt.
Pixels are the smallest light receiving unit (on a sensor) or light emitting unit (on a monitor). Pixels come in various sizes and density. Camera sensors measure them in the millions, spread like salt (only very carefully arranged) on a sensor measured in millimeters. The pixels themselves may be of different sizes, the common range being 5.5 microns (millionths of a meter) to 9 microns. Since they’re very tiny, you can pack millions of pixels (megapixels of them) onto a sensor.
Pixels on a monitor are different. They are commonly given in pixels-per-linear-inch. Despite what you used to hear about 72 per inch (mac) or 96 per inch (PC), most monitors have between 90 and 105 pixels per inch. If your screen actually has a pixel density of 100 per inch, that means you’ll have 100x100= 10,000 in one square inch of screen. Compare that to the millions per square inch on your sensor.
Obviously pixels on a monitor are far more widely spaced and hundreds of times larger than the pixels on your sensor. The space between pixels on a monitor is called “pitch” and is commonly on the order of .27 or so millimeters between each pixel.
I’ll get to the so-what of all this in a moment; but first let’s look at DPI.
DPI – dots per inch – has nothing to do with sensors or monitors. It has everything to do with print quality. DPI is the number of dots of ink sprayed on the paper by an ink-jet printer. For normal print sizes and magazine use, 300 PPI is the common standard for image files you send to newspapers, magazines, and labs like Bay Printers or WHCC; below that, the image simply won’t print well enough for magazine or book use when converted to DPI; art directors are just persnickety that way.*
So size your print file at 300 PPI and you’re golden, right? Well, maybe not. First, very large prints do not need to be printed from 300 PPI files. I’m talking really large – 20 x 30 inches up to by 20 x 30 FEET. For instance, the graphics on airplanes are printed at 25 dpi.
Why? Two reasons. First, the bigger the print, the bigger the file needed to produce it. If you wanted to print a file for printing 20x30 feet at 300 ppi, you’d need a file measured in terabytes! (7.77 BILLION dots). And you still have to convert image data to printer data; watch out!
The second reason is that human eyes aren’t that good. When you watch a TV across a room, it looks pretty sharp – even though the screen pixels are separated quite widely. That’s because your eyes don’t see well enough to see each screen pixel separately; we merge them together and form the image in our brain the way we think it OUGHT to look.
So billboards and airplane graphics don’t need to be printed at 300 dpi or more to look sharp. If you’re seeing them from 150 feet away (and that’s WAY closer than you want to see the tail of an airplane near you), they’ll still look sharp at far lower density. So what if up close they’re awful? Doesn’t matter, and processing files does matter in terms of both time and equipment. So thank heavens for human limitations!
In fact, the further away an object is, the less clearly we can see it – thanks to atmospherics and some laws of physics and physiology that we don’t need to mess with.
So, for ordinary purposes, you should try to size your image to print at your chosen size at 300 PPI, or no less than 250 (which will still look soft).
Wait a minute. Your camera and computer are seeing images in pixels per inch – but your printer is using dots per inch. How do you get from one to another?
The best explanation I’ve seen comes from Josh Lubbers, head of BlueCubit Software, and an old hand in the color wars.
Customer has a 300 PPI image open in their application that they wish to print. They click Print and send the file to the print control panel for their Epson Printer.
Printer Driver resizes the 300PPI image to 720PPI using nearest neighbor (low quality) or Bi-Liner (low quality) interpolation. This happens in 100% of all print jobs that are printed from Photoshop, Lightroom, etc. Why? An inkjet printer has a native resolution that all images must be set to in order to print. The same is true for canon and hp printer but their native PPI is 600 PPI.
Print Driver applies a microweave process that converts the PPI (pixels per inch) to DPI (Dots Per Inch with options for 720, 1440, or 2880 DPI on an Epson printer- Canon and HP microweave to different sizes). Printers don’t put pixels on paper; they print dots on paper and microweaving is the process of converting pixels to dots. There is no Image quality lost in this part of the process but a higher level of microweaving generally results in a higher quality print but a slower print time.
Why can’t you just resize the image in Photoshop (or whatever) to the DPI of your printer? You can, BUT 1) you won’t see a difference on your screen and 2) file sizes grow exponentially, and 3) the printer driver still has to convert to DPI.
So that’s it. PPI are pixels, and are electronic. DPI are ink droplets, and are physical, not electronic. Your print driver takes your image and converts it to instructions to the printer to produce the appropriate DPI for printing.
I can’t close without saying that Josh Lubbers makes ImageNest, a post-script RIP (raster image processor) for Mac OS only, that nests your images (to save paper) and resizes them. Version 3.5 bypasses the whole Epson or HP printer driver thing and uses a FAR superior method of up-rezzing (enlarging), and it also has a built-in sharpening routine that gets applied AFTER the image is sized for printing. As RIPs go it’s inexpensive, and upgrades are always free. Yowzah!
* See “A lesson in humility."