The great DPI vs PPI Debate
I see so much confusion around the topic of dpi versus ppi with not only new photographers, but with those that have been around the block a few times. For those photographers that were never involved in graphic design, I'm going to drop a little history on you first. Then, I'll explain why this distinction is important when you're exporting from Lightroom and Photoshop.
Apple and Microsoft
Back in the day, when the first personal computers were created, their main functionality was Word Processing. They had really awesome monochrome monitors and everything was text based. In 1981 Xerox released the Star, which was the first computer with a bitmap display and a window-based graphical user interface (GUI). Apple followed with the Apple Lisa, which eventually became the Macintosh, in 1983. This was the second computer with a GUI. In 1985 Microsoft released Windows 1.0 which was a GUI that ran on top of MS-DOS. There have been many others but these were the first and Apple and Microsoft have obviously been the only remaining operating systems from that era.
Again the primary function on even these newer GUI based computers was Word Processing. Now you could create documents in WYSIWYG (What You See Is What You Get). This was cool, but some standards had to be created.
PPI or DPI?
Now before we get into the meat of this, lets talk about PPI and DPI. PPI stands for Pixels per inch, DPI stands for Dots per inch. They are not interchangeable although it happens quite frequently.
So Apple decided to standardize on 72 PPI for their display. This was derived from the Point measurement in Typography. We are all familiar with choosing the size of our text by choosing its Point size. 1 point is equivalent to 1/72″. So on an Apple display, 1 point was equal to 1 pixel on the display. Most monitors at the time had a physical PPI of 72 so this was a perfect fit. This was great until you had fonts at about 10 points and then they started to look jagged.
Microsoft chose to implement 96 Pixels Per Inch as its standard. Regardless of the physical restrictions of the monitor (72 PPI) Microsoft chose to treat all screens as though they were 96 PPI. This would allow for greater detail in fonts, but it also made elements on the screen larger. So there was no longer a 1:1 relationship with what you would find on a printed page.
Remember monitors and graphics cards of that time were primarily 640×480 in resolution at 72 PPI. That is about 8.8″ wide by 6.6″ inches tall. So on an Apple you could essentially see the equivalent width of an 8.5″x11″ piece of paper. However on the Microsoft system, at 96 PPI, that cuts your width down to 6.6″ by 5″. There is speculation that this is why Microsoft Word defaults to a 1″ page margin so that you would be able to see the visible printed area on the screen. (Trivia!!)
Much of the confusion also comes from the fact that a Pixel is actually made up of dots. But monitors are hard set at a specific Pixels Per Inch. You won't usually find the PPI referenced these days. They list resolutions, like 1920 x 1200 or 2560 x 1600. If you have a 26″ monitor at each of these resolutions, it will have a different PPI. Remember monitors are measured diagonally, so to figure the PPI you measure actual width and height. So approximately 21.75″ width by 13.5″ high. 1920/21.75=88 PPI, 2560/21.75= 117 PPI.
So what does this mean in the real world? If you have an image that is 100 pixels by 100 pixels, on a 1920×1200 display, that image will be slightly larger than 1″ on the screen, on a 2560×1600 display, it will be slightly less than 1″. Make sense? It all boils down to the math. If you are sending images out to a Pro lab they will have some requirements on PPI.
What about DPI?
DPI is a measurement of a printer. Printers print in dots. Because of the way printers work and how they lay down the colors, the resolution or DPI is always going to be much higher than that of a monitor. As an example, an Epson R2880 Ink Jet Printer advertises a resolution of 5760×1440. That means horizontally it can print 5760 dots per inch. So if you are printing an image that is sized at 300 PPI, every pixel of your image will be printed with approximately 19 dots of the printer head.
Think of it this way, PPI is what you are sending to the printer and DPI is what it's printing out.
Let's talk about this. When a file comes out of your camera, it does not have any PPI. It is just a set number of pixels. That's right there is NO PPI on a RAW file. And guess what? Setting a PPI has ZERO affect on the actual size of your image. Yes you heard it here first (maybe), exporting an image with different PPI settings, will get you the same file size on every image AND it will display exactly the same on a computer monitor. Why?
Because you are not changing the amount of pixels in your image, you are simply giving it a reference that a printer will read to help determine print size.
I hear photographers asking about how to only give a client a 4×6 or 5×7 digital file and they are setting the file to 72 DPI. Let's put the nail in the coffin on this for good, no software allows you to set DPI. Look at the Lightroom Export Dialog
You can see it right there, PIXELS PER INCH.
So what do you do if you want to export for a maximum print size? Well you have to do the math. At 300 Pixels Per Inch, to limit to a 5 x 7 print, your maximum resolution should be 1500×2100 pixels. But guess what? They can print that at a lower resolution and get a bigger print, it might not be great but it may be good enough for them. Say they print it at only 150 PPI, now they are printing a 10 x 14.
You can get a better idea of this in Photoshop. Open an image and then go to Image..Image Size. I have unchecked Resample as we don't want to physically change the file yet. Here you can see that at 240 Pixels Per Inch, this file can print to 23.4 x 15.6 Inches. Notice the dimensions of the image are 5616 x 3744.
What if we change Pixels Per Inch to 10?
Now we could print 561.6 x 374.4 inches. Note that the dimensions nor file size of our image has changed.
What if you want to actually change the dimensions?
Then you turn on the Resample checkbox, and now it will change the actual file size. Here I have typed in 4 x 6 as my final Print Size and you can see it has now reduced the file size and dimensions.
Note: If you use the Save For Web feature in Photoshop, it sets the PPI to 72, otherwise you just set the PPI you want in the Image Size and then Save As and select JPEG as the file type. Save For Web gives you more options when dealing with images for the web.
In Lightroom if you want to change the dimensions of your file then you select one of the options in the Image Size Drop Down.
There are several options here, and most can be changed between pixels and inches. Any changes you make here are essentially doing the same thing as the Resample checkbox in Photoshop, it will change the physical dimensions and size of the exported file.
Below are some images that I have exported with various settings from Lightroom and Photoshop so you can see the effect each setting has. Note that all of these images display exactly the same regardless of PPI setting and file size is the same, with the exception of the file using Save For Web in Photoshop. It seems to do a better job of compressing than just using Save As. As always if you have questions, feel free to contact me.