The Myth of 72 dpi .....by Ian Bateman F.R.P.S.

With more and more people taking the plunge and "going digital", many are experiencing a steep learning curve in preparing images for display on a computer monitor or digital projector. Even people experienced in Photoshop techniques and digital printing will sometimes have to "unlearn" their customary scanning procedure. To be displayed at the highest quality it is important that the images are saved at the optimum resolution for screen display, which is quite different to that required for inkjet printing.

You will frequently hear people saying that "72 dpi" is the best resolution for screen display. Some others will nod wisely, then mutter something like "that’s okay for a Mackintosh, but for a PC it has to be 96 dpi". These statements have been bandied around for so long now that they have become an accepted truth, but in fact neither of them is correct. Regardless of what you may have heard, a resolution of 72 or 96 dpi is quite simply a false concept, and is totally meaningless when preparing an image for optimum screen display. Even Adobe have helped perpetuate the myth. If you use Photoshop’s "Save for Web" feature to save an image and then re-open it, you will see that Adobe have helpfully given it a default resolution of 72 dpi. Why? Because that’s what people expect to see! That doesn’t mean that the resolution is "wrong", its just that for images intended for screen or projector display the measurement is simply irrelevant.

A computer monitor or digital projector displays an image at a fixed pixel dimension. The most common format for home computers is currently 1024 pixels wide by 768 pixels high. Older screens may be set to 800x600 pixels, and large screens could be 1280x1024 or even higher. I’m writing this article on a 21" monitor set to display 1280x1024 pixels. The visible width of the screen is actually 15.9" across, so on a "dpi" basis the screen is actually displaying 80 dots per inch. If I changed the resolution to the more common 1024x768, then the screen display would be 64 dpi. The figure will vary with the type and size of monitor used, as illustrated on the chart below. Only once does "72 dpi" appear, and 96 dpi not at all! The chart is taken from www.scantips.com, an invaluable resource for lots of hints and an even more exhaustive de-bunking of the "72 dpi" theory!

APPARENT resolution of different size monitor screens.

Screen size.	14" screen	15" screen	17" screen	19" screen	21" screen
Resolution	Width 9.7"	Width 10.6"	Width 12.5"	Width 14.4"	Width 15.9"
640 x 480	66 dpi	60 dpi	51 dpi	44 dpi	40 dpi
800 x 600	82 dpi	75 dpi	64 dpi	56 dpi	50 dpi
1024 x 768	106 dpi	97 dpi	82 dpi	71 dpi	64 dpi
1152 x 864	119 dpi	109dpi	92 dpi	80 dpi	72 dpi
1280 x1024	132 dpi	121 dpi	102 dpi	89 dpi	80 dpi
1600 x 1200	165 dpi	151 dpi	128 dpi	111 dpi	101 dpi

So why has "72 dpi" become such an accepted measurement?

In the early days of computing, monitors sizes and resolutions tended to be much more standard, and the concept of "72 dpi" was hence much more valid. Apple actually advertised their early computers as having "72 dpi", but even then the concept was misunderstood. Back then, computers could only display text and basic graphics, and "72 dpi" referred to the font size displayed on the screen. Font faces are dimensioned in Points, and a printer’s Point is 1/72 inch on paper. Mackintosh computers were primarily used for desk-top publishing, and Apple wanted the screen display of their monitors to replicate as closely as possible the format of the printed page. Hence by displaying the font at 72 dpi the document on the screen would be printed exactly as seen.

When Windows was launched, Microsoft chose to have a slightly larger font displayed on screen for easier visibility, and hence used 96 dpi for their font setting. These sizes only refer to the scale that the fonts are displayed on screen, and have absolutely nothing to do with the way that images are displayed. The two figures have become enshrined in popular myth however, and are still quoted today without people having the faintest notion about their actual meaning. The only time that "dpi" is relevant for digital imaging is when preparing an image for printing. Video cards have no concept of dpi, and don’t even know what size monitor is attached to them.

So if dpi is meaningless for screen images, what should you be worrying about instead?

Most of the digital projectors currently being used for digital AV have a screen resolution of 1024x768 pixels. When they project an image with native dimension of 1024x768, then each pixel in the image will displayed exactly as intended. If the original image size is any different, then some compromise has to be made for it to fit on the screen.

For example, let’s say that you have a digital image that you have prepared to be printed on an ink-jet printer. We’ll assume it’s for a horizontal A4 print. Printers work very differently to monitors, and here the concept of dots per inch is much more meaningful. Some articles refer to "ppi" or pixels per inch, and even "spi" or samples per inch for scanned images – both terms are valid, but essentially all three refer to the same thing, so I’ll stick with the "dpi" terminology. The maximum resolution that a domestic ink-jet printer can handle is something like 300 dpi. This will vary by printer, but as a rule-of-thumb "300 dpi" is as good a figure as any. Note that this is a completely different measure than the number of dots per inch that the printer can spray on to the paper. This is the number that tends to be quoted on the printer specification, and is arrived at by calculating the fineness of the ink spray coupled with the speed and positioning of the paper carriage, and is nothing at all to do with the image being printed.

So our A4 image, when printed at 300 dpi, has to have a horizontal width of eleven inches. To achieve this printed width, the image needs to be 300 x 11 pixels across, which is 3,300. If your monitor screen is set to 1024x768, then only 1024 of the 3300 pixels can be displayed. In real terms, each pixel of the display has to represent 3.22 pixels of the image. The computer cannot replicate this, so in practise the video card will discard two out of every three pixels as it scans across the screen, except every fifth pixel it displays it will have to discard one out of two instead. The result is that lines and edges that appear smooth on the original image will now look jaggy when displayed. When projected on to a large screen, the jaggy lines will become more exaggerated and hence look worse.

To achieve the best result, the image has to be sized for the screen rather than for the printer. For a full-screen display, that means that the image has to be 1024 pixels wide and 768 pixels high so that each stored pixel is viewed on a 1:1 basis on the screen. For a projected image the concept of "72 dpi" becomes even more meaningless – if you project an image on to a six foot wide screen, the picture you see will actually be displayed at just 14.2 dpi!

ianbateman@wantage.freeserve.co.uk