Output resolution, i.e., PPI, DPI and LPI is commonly misunderstood. Here is a short, non-technical, non-scientific, simple explanation. (I am sure someone will leave comments about what else could have been said, but this is designed to get people started – if you want more information, it’s readily available with basic online searching skills.)
Terms like PPI, DPI and LPI are used to determine the difference in how resolution is measured depending on device. Many times you will incorrectly hear them used interchangeably. The Byzantine jumble of acronyms and jargon that is used to describe resolution is crazy. But never fear. Just memorize this concept, and you will understand more about it than most so-called experts.
When measuring DESKTOP PRINTER OUTPUT RESOLUTION, use dots per inch. DPI refers to the resolution of an output device like a laser printer or image setter. For example, a standard office laser printer is 300 dpi. A standard image setter is 2,540 dpi.
When measuring PROFESSIONAL OUTPUT RESOLUTION, lines per inch LPI is the resolution (line screen) of a professional printing press, which determines how much detail the press (and the paper) can hold.
When measuring MONITOR/SCANNER OUTPUT RESOLUTION, use pixels per inch. PPI is the resolution (or detail) of an image in a scanning or graphics program, and the resolution of computer monitors. For instance you might want to make it a practice to generate 72 PPI photos for the Web.
This post sponsored by Lensbabies.
Good advice on the 72 PPI for the web, some monitors will display up to 120 PPI now.
Scott, can you either post here or do a segement on the show about best practices for setting these (specifically DPI for printing)? Maybe you could also explain re-sampling an image to get a clearer image for small prints.
Thanks!
CRT monitors were typically 72 PPI. Most LCD screens are 96 PPI though some are 96×95 PPI, and as mentioned can be much higher.
Also please note (especially my clients) image formats such as jpeg, gif and png or indeed RAW have no concept of dpi, ppi or fpi (ferrets per inch). <– I might have made that one up.
They are just a set number of pixels high and wide. It is the output resolution of the thing you are viewing them on / printing on that will determine how physically large the result is.
@Alex not sure this is on topic as the point of the post is to get the nomenclature right.
As I understand it, PPI is only useful when dealing with an output of known physical dimensions, as you need to determine how many pixels you want per physical inch. 300 is usually the most an inkjet printer can reproduce before you get to the point of severely diminishing returns.
Saying people should “generate 72 PPI photos for the Web” is misleading, since as has been mentioned, PPI has no practical relevance when dealing with the web, because monitors come in a whole range of sizes and screen resolutions. When rendering for the web, it’s only meaningful to think in terms of pixels, not inches (or mm or whatever).
DPI is something most photographers need not concern themselves with, as it refers to the printer’s hardware capabilities. Inkjet printers use lots of individual dots to make up one pixel, which is why their resolutions are quoted in the thousands of DPI.
@Andy I couldn’t possibly disagree with you more but have at it.
By the way – you might want to read my post a bit more carefully. If you do, you’ll notice the following sentence…
“For instance you might want to make it a practice to generate 72 PPI photos for the Web.” Note the word MIGHT.
Your comment is misleading as you quoted me thusly…
“Saying people should “generate 72 PPI photos for the Web.”
I never said SHOULD. I said MIGHT because I knew I’d get a comment like this and wanted to forestall the mindless minutia that gets talked about when this subject comes up among pixel peepers.
Again – let’s try to stay on topic which is – understand the nomenclature and don’t interchange PPI, DPI and LPI.
Thank you.
When I look at the Exif information on a picture from my Digital Rebel it has X Resolution 72 dots per ResolutionUnit and Y Resolution 72 dots per ResolutionUnit I’ve always wondered what this meant? And how it compared to a 300 DPI printer? Any light that can be shed on this would be appreciated. Thanks.
Steven, the resolution as it is set in the camera software is somewhat arbitrary. It’s deciding to assign the image a physical size (in inches) of however many pixels you have, divided by 72. In your image editing software you’ll want to resize the image to the dimensions you’d like to print at, at 300 dpi, before printing.