PPI vs DPI

In a nutshell, PPI (pixels per inch) and DPI (dots per inch) are two ways to measure resolution and are very similar in how they operate. Before saying anything more it's worth mentioning that many people unknowingly refer to PPI as DPI so there’s plenty of room for confusion.

First lest talk about DPI. This is the number of printer ink dots in an inch and more is better. It's totally independent of an images number of pixels or ppi and is totally controlled by the printer. Some printers can print at a higher DPI which in theory will make the individual dots less visible and the overall print look smoother. If you remember the old dot matrix printers they had a much lower DPI then most modern ink jet printers and if you looked close, you could see space in-between the dots of ink.

In general, designers are more concerned with PPI. Before I talk about that however, it’s worth mentioning a few things about pixels.

·         The pixel is the smallest unit that comprises an image. There is no such thing as a fraction of a pixel (fun designer trivia).

·         PPI controls how large or small the pixels are displayed. Say we have an image that is 1000 pixels across by 1000 pixels down. You can zoom in and change each pixel's color individually but how large or small it appears when printed is determined by the pixels per inch. If you set the PPI to 10 pixels per inch, then your image will print much larger (100 inches across and down) then if you set the ppi to 100 which would make the image print smaller (10 inches across and down). The obvious effect of having larger pixels is that the larger they are, the move visible they will be to the viewer and the blockier (more pixilated) the image will appear.

What this means is you want enough pixels per inch so that the viewer cannot identify them individually and you will get a nice smooth image. Typically when working on something for print, designers will work at 300 ppi. Anything higher then this is mostly just making your file larger and at 300 ppi, even thin lines will render smoothly which is particularly important if your image contains text. Most of the time for standard printers you would have in a home or office setting, 150 ppi will be plenty to make images without text appear smooth to the human eye.

Screen resolution is even less. You will often hear that screen images are made or saved at 72 ppi. This is a bit of a myth as the screen doesn’t really care about the image’s ppi setting. Where the 72 ppi thing came from has more to do with how fonts display relative to their point size when working in Photoshop but we’ll save discussion on that for another time.:-)

How an image appears on your screen is dependent on three things. First, each operating system such as Microsof Windows, has a ppi setting (often 96 ppi). Sometimes this can be changed though it rarely is and as you would expect, a higher ppi will make the graphics/interface appear/render smaller. This is only part of the equation though as I'm sure you are aware that each monitor can be a different size and have a different setting for number of pixels it displays across and down. If one monitor is a 21inch and another is a 15in and they bother have a screen display of 1200x800 pixels, then images will look larger on the 21 inch display. Finally most software programs that you would view an image in will typically let you view it at various magnifications. So how an image looks on the screen is very hard to predict.

What you need to know if you are making graphics for the screen such when doing website design, is that the ppi you set in Photoshop means nothing. All that matters is the number of pixels across and down on the image. So you would figure out what size most people will be viewing the image at and try to make your image a set number of pixels that does not exceed that and also won't look too small on it either. Understand that you can change the ppi to 1 million and that won't affect the size of the image in a web browser at all (provided you don't have the box checked to resample/scale the image’s pixels in Photoshop).

In summary, it might be helpful to think of ppi and dpi by comparing it to tv’s and the content we play on them. Think about it this way. An old tv show (low res/ppi) viewed on a high def tv (high res/dpi) will still look crappy. On the flip side a high def show (high ppi) will only look as good as the tv it's on. So when printing an image, you still want to be able to print it at 300 ppi ideall. Really good printers (high dpi) will use all those pixels while lower res printers (low dpi) might not. So in a nutshell, if your working on something that will be printed, it’s best to start out working at 300 ppi and not bother worrying about the printers dpi. And if you are working on something for the web, don’t even bother worrying about either.