Why is the web 72 dpi and print 300 dpi?

In early 2008, a great question was posed on a midwest Adobe User’s Group list I belong to:

“A question just came up at the happy hour, and none of us know the answer. Standard resolutions in the web and print world are 72, 300 and 600. None of these are natural squares, which assuming a regular dot layout grid seems… well, confusing?”

I’m going to paste my reply as-is below. Several people found it quite useful but now it largely walled-up inside a Yahoo group archive. Since it’s a largely unedited stream-of-consciousness kind of post, I’d be glad to take questions about any and every aspect of what I’m now posting.

So,why is the web 72 dpi and print 300 dpi?

Original reply (March 2008):

[— start reply —] 72 dpi is a rough conversion from 72 points, which is how many points there are in an inch. There are 12 points in a pica, where we get ours standard font sizes from. A pica the traditional form of  measurement in typography the old school way. The 72 dpi for screen resolution was an early attempt on Macs to get a one to one correspondence between screen resolution and “old fashioned” print measuresments. 12 pixels on a mac screen should be the same as a 12pt font printed out. 72 pixels printed from a mac should be one inch. PCs have always been 96 dpi, hence the reason early desktop publishing and Mac, not PC, was a natural fit. Now, moving on to 300 dpi. This has nothing to do with the screen or monitor or anything. A standard offset press medium quality screen is 133 lpi, as in lines per inch. The next level of quality is 150 lpi. Above that is 175 lpi. It takes more time and accuracy to print higher lpi. Lines per inch refers to the number of dots in a CMYK film output, or today, direct to plate output. Offset printing uses 4 screens typically, of Cyan, Magenta, Yellow, and Black, printed in that order, to build up an image of dots. The dots from the plate are laid down at differing angles – you’ve seen this blown up in lots of pop art. Photoshop even has a filter to simulate what this looks like. Grab a low power magnifying glass and look at anything printed in your house that is color, like a photograph on a cereal box or junk mail. Ok, junk mail is usually 133 lpi because the paper is cheap, and it’s cheap to print 133 lpi. Cheap paper absorbs ink and the dots spread, so you really can’t print better quality than 133 lpi. Try writing on a paper towel with a sharpie to get the idea. Almost there, hang with me! A raster file (photoshop psd or jpg or tiff) has resolution. For the web it’s 72 dpi. But for print, you need higher resolution than that. But, you only need so much. The process of translating the digital file to the “lines per inch” plate of dots (for the CMYK process) throws a lot of information away. How much does it throw away? Well, it throws away anything above about double what the lpi is. If you are printing 133 lpi, then you need 266 dpi. If you go lower than 266, you will start to actually see pixels, the square elements of the digital file, in the final output. So, more resolution = better quality to a point. Anything after 266 or so just gets tossed out. Think of a gravel factory with the big screens that sift rubble into piles. The screen lets small stuff fall through will big stuff makes it to the end. That’s the idea with print. So if you toss sand and gravel along with your hand size rocks, the sand and gravel fall through. If you just put marbles and hand size rocks, the marbles still fall through, and the hand size rocks make it to the end just the same. So why add more sand if it’s just going to be tossed out at the screening process? More resolution just means bigger files, wasted disk space, slower previews in InDesign, and longer film or plate generation times as all that data has to be processed. So, why 300 dpi? Well, 9 times of 10, your final art might just be a tad too small. Maybe you need, after you exported the PSD, 10% more size. Happens all the time. Say there is just too much empty space in a group shot around a group of friends that you just realized 5 mins before you have to send the files to the printer. Should you boot up Photoshop for the edit? Well, if you made your picture at exactly 266 dpi at 100% of the size the image needs to be in the final layout, you really can’t blow it up without degrading quality. But if your image is 300 dpi, you have about 15% headroom where you have “extra” resolution to sop up in the final screen. Therefore, you can safely bump up images 10-15% if it’s 300 dpi at 100%. Now, if you send a 300 dpi image at 5 inches across, but it’s going to be printed at say 2.5 inches in the final layout, you now have TONS of headroom, because it’s effectively 600 dpi because you are reducing the image by 50%. Get how that works? If you had a catalog of images that were all 200% bigger than they needed to be, you’d just be wasting DVDs or FTP time or whatever, plus making the process just slower. The goal is to print 300 dpi at roughly 100% of the final print size. Finally, what is 600 dpi? Simple. The 300 dpi relates primarily to images that are printed in CMYK. The screens and colors make up the final image of tiny dots. But what about line art? Like a pen and ink drawing or a black and white version of a logo? The human eye needs about 600 dpi to be fooled into seeing pixel based art as non-pixelated and nice and smooth with no jaggies. If you print a logo from the web, you can see the dots on the printer. But if you print a logo at 600 dpi, from about a foot away, you really can’t see any dots or edges. But if you zoom in in photoshop, you can see how “bad” it looks. This is why 600 dpi is the baseline for laser printer resolution. Since the final print of line art has no screens involved to help the illusion along, you have to have sheer numbers of pixels to fool the eye. Beyond 600 you really can’t tell too much, but below 600 you can. It’s just that way. Well that was one big brain dump. I hope it helps! Doug [— end reply —]

In conclusion

Yes, indeed, that was one big brain dump. Not an easy article to scan, but is worth the time to read carefully if the resolutions of web and print as of yet still befuddle you.