Here's ppi in your Eye.
Recently someone I work with asked me what dpi was. They stated that they have heard of ppi, but wanted to know the difference and if it really was important to know.
The real answer: Yes, it is.
Its important to know the difference. A way I compare it is like this; your odometer has both MPH (miles per hour) and KPM (kilometers per hour) if your going 70 KPM is that the same as 70 MPH? No, your actually only going 46.6 MPH. Lets look back at our subject, and at the difference between them.
First is is important to know really what the acronyms stand for (according to Wikipedia):
- Dots Per Inch (DPI) – is a measure of printing resolution, in particular the number of individual dots of ink a printer or toner can produce within a linear one-inch (2.54 cm) space.
- Pixel Per Inch (PPI) – is a measurement of the resolution of a computer display, related to the size of the display in inches and the total number of pixels in the horizontal and vertical directions.
From a distance this can be hard to distinguish, you can really notice the differance once you "zoom" in and get a closer look.
First I simulated the apperance of a printed image. Upon closer inspection you can see how the image of the flower shown on the right is reproduced. In this simulation the flower is shown being recreated using 4 colors, typical in most offset printing scenarios, here the colors would be (C) Cyan (M) Magenta (Y) Yellow and (K) Black. It is very simple as the DPI ratio increases the number of dots in produced in the each inch rises as well. For example: an image set at 300dpi would have layed 300 dots in that inch.
To continue showing the difference look at the same image below. In this example the same image is taken and zoomed in to show the pixels that are used in creating the same image.
So really what does this mean? Here's what I told my coworker if the images your working on has a final intent of being only displayed on a monitor somewhere design your graphics in terms of ppi. Typically 72 ppi is adiquate for screen work ( also 96 is becoming very common) it will allow for sharp images and also smaller file size. If your final intent is to have the piece printed then you'll want to work at a higher setting. 300 dpi is a good default setting, talk with your print service provider and ask them. Another benifit is that it is eash to scale a 300dpi image down to 72 ppi.
Hope this helps out.
Update: due to several requests of how to change Photoshop to dpi rather than ppi. You can't do that unfortuanatly many program freely switch the uses of the two phrases. In whatever program you use if all you see is ppi enter the dpi setting all the same.
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