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Are all images on the web 72dpi? Can you use them in print or do they look terrible?
April 21, 2004 8:04 PM   Subscribe

I've heard from 3 different people that you can't use images from the web in print media because all images on the web are 72dpi. Is this true? [more inside]

Aren't there images available in higher dpi (300 is ideal) available on the web? I searched google for images of ancient artifacts, imported one into photoshop, and it appeared to be 300dpi when I checked the image settings. Is this correct?

Also, if dpi is dots per inch, it seems to me if you have a large photograph at 72dpi, you could somehow transform it to 300dpi (is this what resampling is?), the image would just be smaller, yet would compress more pixels in a smaller area wouldn't it?
posted by banished to Computers & Internet (26 answers total)
 
The images only display at 72dpi so most people will reduce them to that to save bandwidth, not everyone does though.
posted by Mick at 8:15 PM on April 21, 2004


If it's 72 dpi, you can't use it in print. If it's 300 dpi, you can.

Whether or not it appears on the web doesn't matter. I don't know the entire explanation behind why you can't use a "really big 72dpi image" and resize it smaller, but I've tried it, and it doesn't work.
posted by rocketman at 8:16 PM on April 21, 2004


72 dpi is as much as you can usefully reproduce on screen. Meaning, if you have a 300 dpi image, it won't look any better than a 72 dpi one, on screen [example]. There are a lot of images online that are 300 dpi, however, Photoshop is not lying to you. If you take a web image at 72 dpi and resample it to 300 dpi, exactly what you say will happen will happen: smaller image as far as print size, more pixels in an inch. I seem to recall there was a huge thread about this, and corresponding file sizes at some point in the past, but you seem to have a handle on it. Your friends are the ones who are a little off, though generally right, most web images are 72 dpi, not all of them are. If you find one pictures at 72 dpi, esp if it's a photo of something old and arty, it might be worth clicking around and seeing if you can't find another one at higher dpi.
posted by jessamyn at 8:18 PM on April 21, 2004


It sounds like a broad claim to say all images "on the web" are at 72dpi. After all, I've scanned images at 300dpi at put them straight on the web, thus immediately disproving their theory. And, like you said, if an image is large enough, you can scale it down and the resolution will be fine. What, afterall, does "dpi" mean on a wide variety of computer monitors? How can an image be "72dpi" on my 15" laptop and on a 21" screen? Or at 640x480 vs. 1280 x 960?

I'm sure people who deal professionally with images will help you out more in this thread, but really, the size and resolution of images can be anything on the web, and while a lot of images are quite small and crappy, that doesn't mean they are all unusable in print.
posted by Jimbob at 8:19 PM on April 21, 2004


An image itsself isn't a specific number of dots per inch, it's a number of pixels (i.e. sampling points) (though some image formats save the dpi in the file metadata, this usually isn't very relevant for images on the web. How many inches big the picture is going to turn out is almost always just a function of the resolution and monitor size of the client).

One of the problems I can imagine with taking a big picture from the web and printing it small is that because things will get a lot smaller in the average case, a lot of small details will fuzz up the picture. Try filtering out the high frequencies (with a blur or smooth filter) and printing it again.
posted by fvw at 8:21 PM on April 21, 2004


The 'dpi' Photoshop gives you really means nothing. A pixel is a pixel is a pixel. What really matters is the number of pixels are in that image, and how big you want it to be in print. Take the number of pixels in one direction and divide by the number of inches it'll be in your image. That is going to be the real DPI. If it's >300, you're golden and it'll work great in print. If your layout software is dumb and will resample the image down to 72dpi if the file says it's 72dpi, just go into photoshop and change that value to 300, and uncheck the 'Resample' box.

If the 'real' dpi is well under 300dpi, it'll look bad, even if you resample it. It'll just look blurry.

On preview, what fvw said, too.
posted by zsazsa at 8:22 PM on April 21, 2004


Thanks a ton for the help guys, this helps out a lot.
posted by banished at 8:27 PM on April 21, 2004


jessamyn: The numbers in that link are way off. The screen I'm currently behind is approximately 100dpi, and it could go a lot higher if I ran it at 1600x1200 or was using a smaller monitor. Also, they neglect to mention that to drive a normal projector screen at 72dpi you'd have to run it in a 7200x5400 pixels video mode, which is probably beyond everybody's budget. Comparing screen dpi's to print dpi's to projection dpi's is a senseless waste of time anyway.

Also, using "higher resolution" images in websites does help the page look better if people scale it up with their browser.
posted by fvw at 8:27 PM on April 21, 2004


Jessamyn, that example site is kind of lying to you. If you actually look at the two images (300dpi, 72dpi), you'll see that one has wayyy more pixels than the other (though you can tell they blew up the 300dpi version and it's a little blurry). They forced the size down in the browser with width= and height= attributes in the img tag.
posted by zsazsa at 8:28 PM on April 21, 2004


They forced the size down in the browser with width= and height= attributes in the img tag.

Which is kind of the point, as I interpret it...what matters is the number of pixels, as you said, not the "dpi". The link sort of demonstrates the relationship between physical image size, scaling, and "dpi". On a screen, the number of resolved dots is fixed (100dpi, 75dpi, whatever) so the scaled image looks the same as a lower-res, non-scaled image. In print, however, where the number of dots is potentially higher, the scaled 300dpi image would look better than the non-scaled 75dpi image. But, on a screen, they're the same.

I guess what I'm trying to say is that yes, the demonstration is pointless, but so is comparing dpi on the screen to on paper.
posted by Jimbob at 8:38 PM on April 21, 2004


Some of the designers where I work have suggested you can actually get away with a DPI in the 220-260 range, depending on the printing process and linescreen.

Does anyone know if the selection of 300dpi for 150 lpi linescreen printing is related to the Nyquist theorem -- ie, the same kidn of choice to sample audio at 44khz because the limit on human hearing seemed to be 22khz?
posted by weston at 9:02 PM on April 21, 2004


I've always heard 1.5 times linescreen works okay for printing photos, but I round up to 300-400 in my work.
posted by MegoSteve at 9:08 PM on April 21, 2004


Yep, 1.5 X linescreen. Newspapers will typically print at an 85 line screen which means photos don't need to have any more resolution that about 130 DPI. Magazines are (if memory serves) around 110 or 130, it varies. I'm not sure about off-set presses, but frankly, we typically have more resolution in our photos than we need, so we just set them at 300 DPI and call it good. 300 DPI also looks pretty good on a laser printer or inkjet which is probably why a lot of people say that you need 300 for printing.

You can print at as low a resolution as you want. It won't look as sharp though if you don't have enough resolution.

There was another thread on the differences between pixels per inch, dots per inch and different ways to try to make screen captures print out well. I leave it as an execrise for the reader to go searching for it though. You might try a keyword like manuals.
posted by willnot at 9:20 PM on April 21, 2004


We print at 170 linescreen, and use 300dpi images for our mags. I always thought 170 was standard.

this could be helpful for you, banished.
posted by amberglow at 9:38 PM on April 21, 2004


It's important to note that it should be 300 dpi at the actual size you plan to print it. I have seen folks stretch a 300 dpi image 200 or 300% and wonder why it looks bad. Whenever I scan a hard copy or build a new image in photoshop, I always scan/build a 300 dpi image at or above the size I plan to eventually print it.

However, we take 72 dpi digital photos all the time and they still work for our print projects, because we have the camera on the highest settings. Try opening a big digital photo in PhotoShop. Then go to Image > Image Size and make sure "resample image" is unchecked. Type "300" in the resolution box and note how the image size changes but the file size doesn't. These new proportions tell you how large that photo can run in a print project without looking bitmappy.

as zsazsa said, it's all about the pixels.
posted by whatnot at 9:41 PM on April 21, 2004


DPI is overrated. What really matters are the number of colours.

Ever seen the output from a dye-sub printer? Looks like a bleedin' photograph... and the resolution is what most people would think atrocious: something in the 100-150dpi range.
posted by five fresh fish at 9:42 PM on April 21, 2004


Weston:

Linescreen is related to Nyquist, but not directly for all processes.

When I was working in the prepress industry, I did some tests with the following dpi/screen combinations on CMYK and grayscale photos:

1) 300/150
2) 266/133
3) 150/150
4) 133/133
5) 300/stochastic
6) 266/stochastic
7) 150/stochastic
8) 133/stochastic

When we did contact sheets to see the results, numbers 1 and 2 (CMYK) were virtually identical, even with a loupe. 3 and 4 (CMYK) didn't look bad, but they fell apart in the detail a little. 3 and 4 (Greyscale) looked pretty bad. 5-8 all looked amazing - all as good or better than even 1 or 2.

The major problem here is that while Nyquist is directly relatable to the process of taking a continuous tone single-channel image and converting it to different sized dots with a linescreen, Nyquist doesn't directly take into account that CMYK adds a bunch of bandwidth.

Stochastic screening is the coolest process evar, akin to adding noise to the oversampling stage of your CD player - it makes even low resolutions look passable. Our RIPs had just started to support this, which is why we did the tests.

Obligatory YMMV: We only tested a handful of different images, and then once we chose our new process, we had to adjust ink densities on the presses.

Back on topic though, images don't have a DPI - output devices do.
posted by tomierna at 9:59 PM on April 21, 2004


I don't know the entire explanation behind why you can't use a "really big 72dpi image" and resize it smaller, but I've tried it, and it doesn't work.

This probably has to do with jpeg compression. Outputting an image as a jpeg does not render a re-usable image for just about any purpose. The image is optimized for a specific size, quality setting, color pallette, etc, and then it's crunched through a series of algorithms and spit out at those settings, optimized for use at those settings. It's now a series of mathetmatical expressions, not a series of pixels, and it can't be manipulated as easily as a bitmap or other un-compressed format. This is probably why shrinking 72dpi web jpegs down doesn't look so hot in print. You're compressing and already compressed image.

An analogy would be: if you want to translate a French play into 12 languages, you wouldn't first translate it into English, then work from the English version and translate that to German, then take the German and translate it to Arabic, etc. Each translation is only an approximation, a near miss, and doesn't retain the flexibility and solid-centeredness of the original.
posted by scarabic at 10:09 PM on April 21, 2004


Wow. Thanks, tomierna.
posted by weston at 10:12 PM on April 21, 2004


The reason you don't use web images in print is because (most) print is a higher resolution output format than a display. This is why images that look OK in, for example, your browser appear blocky and crappy when you print a page. In order to print to an equivalent size, each displayed pixel has to be composed of a shitload of printer dots.

The whole "72 dpi" thing is misinformed: it comes from print people who don't understand the concept of pixels; they're used to having some kind of mapping between data and physical output size, and unaccostomed to conceptually addressing the output on a per-dot basis in the way that, say, a graphics programmer would.

Of course there are graphics on the web that would be suitable for print use. Just not most -- or damn near all -- of them. Few pages make use of 8000 x 8000 bitmaps.
posted by majick at 10:24 PM on April 21, 2004


The whole "monitors are 72 dpi" thing is actually about fonts, not images. Font sizes are specified in points, and points are 1/72 of an inch. What 72dpi means, in regards to computers, is that a 72 point tall letter, which will print at one inch tall, will display on the screen as 72 pixels tall. Note, this says nothing about the actual resolution in terms of dots per inch in either medium. The printer will print the letter one inch tall and at whatever resolution it's set to print at, and the screen will display the letter 72 pixels tall, which may be one inch or half an inch or three inches, depending on your monitor size and display settings. Make sense? So think of monitor "dpi" as a number that tells you the relationship between pixels on screen and points on paper. With images, there's no difference between pixels on screen and dots on paper. Put the dots closer together (higher dpi) and the image is smaller. Put them further apart and it looks blockier or blurry.

Anyway, the main thing is that what zsazsa said is completely right.
posted by Nothing at 11:04 PM on April 21, 2004


DPI doesn't have anything to do with bitmap images. Bitmaps have no relationship to real-world dimensions.

It's not until you try to place an image within some physical dimensions that you know how many pixels per inch there are, and it's simple math.

The idea that "monitors display 72 DPI" is wrong. My screen is at 1600x1400 at 16x14 inches wide, so I can display at 100 DPI. If I change my resolution down to 640x480 then I'm only at 42 DPI.

Ok, I need to go now.
posted by holloway at 12:03 AM on April 22, 2004


I was editor in chief at an English language newspaper in Budapest run by insane nouveau riche Russians. After firing all my staff and freelancers I had to write the entire paper myself, while the Russian owners specifically ordered me to steal images and stories off the internet because "that's how we do it in Russia." The images were acceptable in a small format, but were obviously trash when blown up to a half page for the cover, which we did a lot. The owners didn't care. Every stolen image was money saved. I was called to the carpet for asking permission to use World Cup Soccer stories and images from the Japan Times - and the receiving said permission. ("Bad precedent.")

It was the worst job I ever had, and I have been a lab animal caretaker and a garbage man in Hackensack.
posted by zaelic at 1:08 AM on April 22, 2004


Incidentally, I usually change the dpi of my images in Photoshop by changing the dpi setting and then going back (before hitting "okay") and changing the size back to its original. It's stupid of me not to just uncheck the "resample" box, huh?

What tomierna said: only output devices have dpi. Damn straight. Of course, this indicates that monitors have dpi, and they do, but as Nothing says, in that context the traditional nomenclature means something else entirely. But at a given resolution, your monitor does have a definite dpi. But what about its geometry?
posted by Ethereal Bligh at 1:26 AM on April 22, 2004


What's stochastic screening? It sounds great!

FWIW, we've always used the "two pixels for every line" rule - 150lpi gets 300dpi images. But that doesn't stop us stealing headshots found on google images all the time. Just whap on the "add noise" and it's all good.
posted by bonaldi at 4:04 AM on April 22, 2004


stochastic screening.

Basically it introduces a "randomness" to the dot (no set angles). Which avoids a moire and allows for a smoother image at a lower lpi.
posted by rschroed at 6:39 AM on April 22, 2004


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