I first wrote my article explaining the difference between DPI and PPI around 10 years ago. To put it in perspective, it originated as a post on Usenet. I posted that article mainly to help clear up the difference in the terms because I was frequently seeing them misused. Way back then I'd always planned a follow-up article on what I like to call the "72 dpi myth". This week's Tech Tuesday instalment seems as good a time as any to finally complete the follow-up piece to that article. Okay, so what the heck is the 72 dpi myth? It's really more of a collection of incorrect ideas about resolution and how it relates to screen display. One of the more common places you'll see it is people recommending to save your images at 72 dpi when posting online. The usual reasoning for this is so that the images are only good for screen display and don't have enough resolution for a good sized print. There are other variations which often quote 72 dpi as the resolution that all images are displayed on screen.
First things first, strictly speaking we're talking about pixels per inch (ppi) not dots per inch (dpi). I'm calling this the 72 dpi myth because that's how I most often hear it. But this post isn't about the terminology, I already have an entire article devoted to that. My concern today is how this myth perpetuates a misunderstanding about resolution. At best, this spreads ignorance and confusion. At worst it can lead people to think that their web images are too low resolution to be of any use from a would-be image thief when they're actually posting very high quality images.
Readers of my DPI vs. PPI article should already understand the issue here; pixels per inch only matters when you're printing. On screen the only thing you have to worry about is the pixel dimensions. The ppi setting does absolutely nothing on-screen! If you're concerned about posting images that can't be printed very large then all you need to do is worry about the pixel dimensions. Do some quick math to figure out what size you could print the image at an acceptable quality. I'd say 150 ppi is about as low as you can go and get any kind of reasonable quality for a smaller print. So with that figure in mind, an image that's 600 pixels on the wide side would only end up being 4" on the long side. I don't think most people are too concerned about people making postcard-sized prints so this can be a good starting point. I'm not trying to throw out any hard and fast rules though, I post my pictures on my stie at larger sizes than that and I'm not overly concerned. Some people are worried even about this size and won't go above 400 pixels on the long dimension. Figure out your comfort level and go from there.
My point here is that on the screen it's all about pixel dimensions, not the ppi setting. The ppi setting is just a small piece of information that goes along with the file to say how large the image should be printed. However, that can be changed at any time without any loss of quality as long as you're only adjusting the ppi setting and not re-sampling. For example, if I were to post a full 12 megapixel image from my D700 it's not going to matter whether I set the ppi to 72 or 300 or something even higher, it's still a full-size image!
If this is still unclear let's look at this from the other direction, screen displays. The statement that all computer screens display at 72 ppi is dis-proven with a simple examination. Look at standard monitor resolutions; in the old days of 4:3 monitors you had resolutions like 1024x768, 1280x1024 and 1600x1024. In these days of wide-screen displays you see resolutions like 1440x900, 1920x1080, etc. Notice that in all of these examples I'm only talking about pixel dimensions, not the size of the screen. Suppose you have 2 wide-screen monitors, a 19" and a 24", if both are using the same resolution of 1440x900 then clearly the larger monitor is displaying at a lower resolution. Perhaps the best way to really hammer this home is with televisions since they're displays just like computer monitors. "Full HD" is 1080p (1920x1080). Walk into an electronics store and find the smallest 1080p TV you can find, it's probably going to be somewhere in the 30" sizes, now compare that to a 60" TV or bigger that's still displaying at 1080p, or how about some 100" projection screen that's 1080p too. Are you honestly going to say that all of them are displaying at 72 ppi? Clearly they aren't.
More recently I've even stumbled across a new version of this myth that's specific to the iPhone and iPad where higher resolutions (132 ppi for the iPad, 326 ppi for an iPhone 4G) are recommended for these displays. Hopefully by now you can dismiss this advice based on your understanding of resolution. Again, all that matters is the pixel dimensions. If you're concerned with iPhone or iPad display then just go by the pixel dimensions and figure out what you want to use from there (remember though that higher resolutions will allow for zooming). I think some of the confusion here comes from Apple publishing the ppi specs for their displays but recognize that's only useful for comparing with another device's display if you're interested in which display has the higher resolution.