The Myth of 72 PPI

You've no doubt heard the advice: Images for web pages and other monitor presentations must be prepared at 72 ppi (or, less accurately, "dpi," but I think we're all clever enough to know what we mean in either case). The advice, sometimes presented as an absolute requirement, is unnecessary and unfounded. Many people who should know better perpetuate this 72 ppi misunderstanding, so if you're laboring under this misconception, don't kick yourself. You've probably been led astray by the advice of someone who otherwise seems to know something about digital imaging. If you don't want to wade through my explanation of why it's false, just take a look at the two identical images below. Those who remain unconvinced after seeing this demonstration should perhaps consider a career in politics. (Other examples: the button on the left is saved at 1 ppi. The Photoshop dialog box near the bottom of this page is saved at 30,000 ppi.)

Let's define some terms. "Monitor size" is the physical dimension of the display area. We can measure it. The diagonal measurement of the display area is generally used to compare sizes among monitors, with 17", 19" and 21" displays being most common. "Monitor resolution" is the number of pixels the monitor can display. This is given as HxV, where H is the maximum number of pixels displayed in a horizontal line, and V is the maximum number of pixels displayed in a vertical line. The most common resolutions as I write this are 800x600 and 1024x768. The "operating resolution" of a monitor is the number of pixels it displays per linear inch (horizontal and vertical).

The essential problem with the 72 ppi rule is that monitors have no defined standard linear measurement in terms of pixels. We can define the monitor size: 17" diagonal, 19" diagonal, and so forth. We can define the monitor resolution: 1024x768 pixels or 800x600 pixels, for instance. But pixels per inch? That varies, and it's dependent on the physical size of the display and the number of pixels sent to that display, i.e., the operating resolution.

Operating resolution is, therefore, not a constant across monitors. It varies with monitor size and resolution. Consider a 17" monitor that displays 800x600 pixels. It's display width will be about 13 inches. 800 pixels spread over 13 inches yields about 62 pixels per inch. The same monitor, displaying 1024x768 pixels, will yield about 79 pixels per inch. Cleary, 72 pixels will cover more than an inch on one and less than an inch on the other. Similar variations will be observed by keeping the monitor resolution constant and varying the monitor size. Thus, the premise of the 72 ppi dogma is out the window: all monitors do NOT display 72 pixels per inch. A few might, but there is no display constant of 72 ppi. Furthermore, there is no "standard" monitor that displays 72 ppi.

With that out of the way, let's consider whether we can improve the appearance of images we post on the 'net. We might have a picture that's 300 pixels wide and 230 pixels high. If it looks jagged, or "pixelated," perhaps we need higher resolution. Makes sense, right? More pixels means more data and more detail. And since we've thrown the requirement of 72 ppi out the window, which is good, we can now feel free to boost the resolution to 300 ppi if we want -- and we can be confident that it won't matter. Hooray! We can also be confident that it won't improve the image, either. Bummer.

Why is that? Well, the image is 300x230 pixels, and that is exactly how many pixels it will fill on the monitor, regardless of how many "pixels per inch" we've set for the image. So, unless we also change the pixel dimensions of the image (to something other than 300x230), changing "pixels per inch" won't affect how it looks on the monitor. [Note: there are a few software programs that interpolate and resize images based on ppi information. They do this by changing the number of pixels displayed, not by changing the linear density of the pixels. Web browsers, incidentally, are not among them.] We increase the resolution of the IMAGE by raising the overall pixel count. We increase the resolution of the image PRESENTATION by increasing the pixels per inch. But since monitors display image data only at operating resolution, we can not squeeze more data, pixels, into less space on a monitor.

The Evidence Against 72 ppi Restrictions

Okay, so there they are. On the left is 300x230 pixels at 72 ppi. On the right is 300x230 pixels at 300 ppi. On a computer monitor they are identical, and probably not displayed at 72 ppi and certainly not 300 ppi. Instead, they're both displayed at the operating resolution of the monitor. But send them to a printer! The one on the left will be just over four inches wide and the one on the right will be one inch wide. Printers can, and do, adjust linear pixel density based on the dpi information associated with the file. Computer displays ignore dpi information of the file. The only thing affecting the size of an image on the monitor is the number of pixels contained in the image sent to the monitor.

Two things should be evident from all this. First, ppi is irrelevant for web display and nearly all other monitor applications, including digital projection. That frees us from the myth of 72 ppi. Second, if you want more detail in a digital image you've got to have more pixels. The ppi information attached to a file does not enhance or degrade the image data. Keep in mind, however, that on a monitor, more pixels requires more screen space. The push for more detail is what motivates the quest for digital cameras with more and more pixels, but the advantage is rarely realized with the limited pixel counts of computer displays.

With this knowledge, we can simplify preparing images for web sites and other monitor display applications. There's only one decision to be made: How much screen real estate do we want the image to cover? The answer is decided in pixels. Not pixels per inch; just pixels. So we open the image in Photoshop or similar software, and size it appropriately. My dialog box looks like the one on the right. To get my images at 300x230 pixels for this web page, I need to change the Pixel Dimensions width from 828 pixels to 300 pixels, and the Pixel Dimensions height from 465 pixels to 230 pixels. I can ignore the Document Size information that indicates dimensions of 0.207 by 0.159 inches. I can also ignore the 4000 pixels/inch resolution (this value comes from my original scan, and the inches were real linear inches of the film). I leave Constrain Proportion checked because I don't want to alter the aspect ratio of the image. I check Resample Image, and for most of my work I use the Bicubic algorithm.

The Constrain Proportions option limits our sizing options. When that box is checked, setting the Pixel Dimensions width results in an automatic adjustment of Pixel Dimensions height. If you then change Pixel Dimensions height, the Pixel Dimensions width will change. Decide which dimension is most important and let the other adjust automatically, or crop the image so that it's aspect ratio matches the width and height dimensions you want to display.

If you are doing the original scan of an image, I recommend using the maximum native optical resolution of the scanner. That will yield the best scan, and you can always re-sample as required for various applications later.

So now you know. Next time someone starts in about 72 ppi, you can say "har-de-har-har," make wagers all around, then offer two identical images as I have on this page and collect your bets. It's not the ppi that matters. It's the absolute number of pixels wide and high that determine image size on computer displays.

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