DPI vs PPI

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DPI vs PPI

Postby jamesfc on Tue Apr 28, 2009 12:00 pm

No, this isn't about the fight of the century. These two terms seem to cause a lot of confusion among photographers and hopefully I can help alleviate rather than add to the confusion. PPI stands for "Pixels per inch" and DPI stands for "Dots per inch". At first glance they appear to be the same thing, but really are quite different! Unfortunately, many people, including some experts, use the terms interchangably which only adds to the confusion.

PPI (pixels per inch) really should be applied to electronic media such as computer monitors, scanners, etc. It really refers to the number of pixels that occur in an inch of an image display or in the case of a scanner, the number of photo sensors per inch on the scanning arm. In the case of a monitor, especially LCD monitors, this is really a meaningless measurement.

Say you have a 22" LCD monitor that has a native resolution of 1280 x 1024. That measurement means on the horizontal axis there are 1280 pixels. Let's say that that 22" inc monitor (which is the diagonal measure of the monitor size) has a width of 20" - just for the purpose of illustration. That means each inch of horizontal monitor, there are 64 PPI or pixels per inch. So what? It really doesn't tells us a thing. In the case of a scanner, it at least gives us some idea of the amount of detail the scanner is capable of scanning.

Let's turn to DPI. DPI is really a measure of the dots on a printed page. Offset printing is normally done at 300 DPI and our photo printers normally produce 360 DPI or dots per printed inch. Some may go as high as 720, but hopefully you get the idea. DPI is also a measure that is embedded into digital images. This is an instruction that tells the software how many pixels to represent on an inch of printed output. That measure ultimately effects the size of the printed image.

Unfortunately, as I stated before, many use these two terms interchangeably and I have seen it written over and over again that an image produced for the web or email should be saved at a PPI or DPI of 72. This is NOT correct as it really is irrelevent to displayed images! The only thing that matters for digital display is the number of pixels horizontally and vertically. An image that is 1280 x 1024 will fill the screen mentioned earlier pixel for pixel when display at 100%. An image that is 640 x 512 will have two pixels spread over a single display pixel if viewed at full screen (200%). At 100 percent, it won't fill the screen. The DPI number embedded it the image is simply ignored, whether it is 72 or 10,000 - it simply does not matter. This has come about as a mistaken idea that since monitors average around 72 PPI, that the image should have 72 DPI embedded for the best display. Again, this is simply wrong.

Now take that same 1280 x 1024 image and send it to a printer with 72 DPI embedded in it and the resulting image will be 17.78 inches x 14.22 inches. At 300 DPI (offset printing standard), it will be 4.27 inches x 3.41 inches. How do I know? Simply divide the DPI into the number of horizontal and vertical pixels.

So hopefully you now see that DPI only effects the printed output of an image and has no bearing whatsoever on the displayed output on a monitor. Unfortunately, many instructors mistakenly use the DPI measure to force the software to resize an image. This is rather silly. Also, I see over and over again DPI and PPI used interchangeably, when there are really two different concepts. Now that you are enlightened, perhaps you can straighten out the next person who misuses these two terms!
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Re: DPI vs PPI

Postby daisyallen on Tue Jun 01, 2010 5:10 am

IMO, it's much easier to work with print resolutions if you simply multiply the size of the print by the printer resolution.I encounter a lot of people who think a 300 ppi file is always bigger than a 72 ppi file. They often have difficulty grasping that ppi makes no difference when working on screen. It only matters when you're ready to print.
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Re: DPI vs PPI

Postby jamesfc on Tue Jun 01, 2010 8:50 am

You are so correct! I have had instructors who don't seem to understand PPI and DPI and I constantly see advice given to make sure your web or email images are at 72 DPI. I'm assuming these people are using Photoshop to change the image size by leaving the option checked to resize the image to proportionately match the DPI, but the DPI has nothing to do with the image size! Uncheck these options the next time you are in Photoshop and change the DPI to 10,000 and you won't see any difference in the image size and if posted on the web, it won't change either!

Thanks for your posts!!
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Re: DPI vs PPI

Postby aprillove20 on Thu Nov 18, 2010 1:11 pm

IMO, it's much easier to work with print resolutions.
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Re: DPI vs PPI

Postby jamesfc on Fri Nov 19, 2010 5:17 pm

aprillove20 wrote:IMO, it's much easier to work with print resolutions.


I agree - especially if printing your images or working with a printer company. For the web - it really doesn't matter!
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Re: DPI vs PPI

Postby georgesmith on Mon Feb 21, 2011 7:00 pm

DPI is absolutely a ad measurement of the dots on a printed page. Offset press is commonly done at 300 DPI and our photo printers commonly aftermath 360 DPI or dots per printed inch. PPI absolutely should be activated to cyber banking media such as computer monitors, scanners, etc.
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