The Truth About Resolution

When I became a pixel pusher years ago, one of the toughest concepts to wrap my brain around was that of image resolution: What the heck was it, how did I change it without trashing my image, how much did I need when, and so on.
I acted like I got it, but what I’d really done was memorize a couple of magic numbers: 300 for print and 72 for screen. Then I needed to change the resolution of an image. Suddenly, I was face to face with A Serious Resizing Situation and no clue what to do.
I know better now, but many professional designers don’t. They buy a high-quality photo from a stock agency, open it in Photoshop, and black out with rage when they see “resolution = 72” in the Image Size dialog. An irate call to customer service usually ensues, followed by an embarrassing explanation of Resolution 101 by someone half their age. The poor customer service rep hangs up, tells the story to the whole office, and the designer is dubbed a brainless prat. It’s not pretty.
I’d like to uncover the truth about resolution and protect you against such horror. I’ll explain exactly what resolution is, how to change it without altering image quality, how much you need when, and (gasp!) when it doesn’t matter at all. I’ll also talk about increasing resolution, otherwise known as The Dark Art of Upsampling.
Resolution and Pixels Defined
Resolution, when referring to an image, is the number of pixels displayed per unit of printed length. It’s a measurement used in printing and it’s stated in dots per inch (dpi). This makes perfect sense because printers print dots, and that’s what a printed image is composed of.
When referencing an image onscreen — on a computer monitor, TV, plasma, or projector — resolution is stated in pixels per inch (ppi). This too makes sense because digital images are displayed in teeny, tiny individual blocks of color called pixels.
How They Work Together
The resolution measurement dictates how closely the pixels are packed together. Increasing an image’s resolution means the pixels will be packed together more tightly, resulting in a smaller physical size, but generating a smoother, higher quality print. Lowering an image’s resolution means loosening the pixels, resulting in a larger physical image size, but generating a blocky, lower quality print.
Think of the resolution measurement as density. For example, the tighter a substance is packed, the denser it is and the less surface area it takes up (like brown sugar). The more loosely a substance is packed, the more surface area it consumes and it becomes less dense.
The confusing part is that when it comes to imagery, printers are the only devices that can do anything with the resolution measurement. Because our eyes can only process so much information, a 72 ppi image onscreen looks identical to a 600 ppi image onscreen. However, a printer isn’t hampered by the human eyeball and can take advantage of resolutions much higher than 72. (Actually, scanners can, too, but that’s a story for another time.)
How Much Do You Need?
The resolution necessary for a beautiful print depends on the printing device itself. For instance, consumer inkjets do a nice job at 225 to 250 dpi, while professional service bureaus require 300 dpi and higher for glossy magazines, coffee table books, and the like. For a color advertisement in a newspaper, you need between 150 to 200 dpi. Same thing for a black and white laser printer. However, to know for sure, you’ve got to run some tests. If someone else is printing your project, ask what resolution they want.
If you’re dealing with images that will never be printed (Web, email, and onscreen presentations) you don’t need to worry about resolution at all; it’s the pixel dimensions that matter.
Resizing Images from Digital Cameras
Let’s put theory into action on a photo from a high-end digital camera.
Step 1: I opened Figure 1 in Photoshop, then chose Image > Image Size (Command+Option+I/Alt+Option+I).

Figure 1. This is the place to learn about the pixel dimensions and current resolution measurement.
This gorgeous photo (courtesy of iStockphoto.com/Lisa Gagne) is measured at 72 ppi. Does that mean it’s a low-quality image? Negatory, good buddy.
First of all, look at the document file size: it’s 14.1 MB. That’s pretty darn big, which means we’re dealing with a pile o’ pixels. This information is revealed at the bottom of the Photoshop document window, which is circled in red.
Second, the pixel dimensions shown in the Image Size dialog (also circled in red) are 2716 x 1810. Yep, that’s a pile. Remember those numbers for a minute.
Third, note the honking big physical dimensions of the photo, shown in the center of the Image Size dialog. If you tried to print this sucker at its current resolution, you’d need a piece of photo paper more than 37 inches wide by 25 inches tall. Yikes!
Step 2: At the bottom of the Image Size dialog, uncheck the Resample Image option. This magical, all-powerful option locks the pixel data, thereby locking image quality. Watch what happens in Figure 2 when you enter 300 into the resolution box:

Figure 2. Now the numbers are way different.
Did you see the physical size of the image change? It decreased to roughly 9 x 6 inches — a bit more manageable, yes? However, take a peek at the pixel dimensions toward the top of the dialog. They didn’t change, did they? That, dear grasshoppers, is the power of the Resample Image option. I just successfully changed the resolution without altering image quality.
I can prove it in two ways:

  1. The number of pixels is exactly the same as it was in step 1: 2716 x 1810. But now the pixels will be packed more tightly together when the image reaches a printer.
  2. The file size is still 14.1 MB, which is further proof that the pixel data didn’t change.

This is hard to grasp because your eyes perceive absolutely no change in the image onscreen. The difference can only be seen in a print or revealed by the numbers in the Image Size dialog.
Now, if you wanted to resize this image for email or to post on a Web site, you’d want to leave the Resample Image box checked because you’d dang sure want to reduce the number of pixels in that image, or else it’d take an hour to download and that would be very, very bad.
The Evils of Upsampling
If you leave the Resample Image box checked and increase the resolution of an image, you’ll be adding information (pixel data) to the image that wasn’t originally there. It’s usually an extremely bad idea, but there may come a time when you have no choice.
For example, maybe you’ve grabbed an image from the Web that you have to print, or your image will be printed in an extremely large format (like a billboard). If you find yourself in such a pickle, you’ve got a couple of options. Happily, the first one is free.
Evil upsampling method #1: Open the Image Size dialog in Photoshop, take a deep breath, and leave the Resample Image box checked. (I realize I just told you not to do this, but bear with me.) Choose Bicubic Smoother from the pop-up menu to its right and change the document dimension pop-up menus to Percent. Enter 110% in the width box and press OK (Figure 3). Repeat this process as many times as necessary to enlarge the image. For some reason, adding data 10% at a time doesn’t cause a huge amount of quality loss. Do resist the urge to increase the size more than 10% at a time, unless you want that chunky look.

Figure 3. Resist the urge to enter a number higher than 110 in the circled boxes!
Evil upsampling method #2: Buy a plug-in, such as Genuine Fractals by onOne Software, Blow Up by Alien Skin, or PhotoZoom Pro 2 by BenVista. They pull off some serious voodoo and somehow manage to increase pixel data without totally destroying the image.
I Wouldn’t Lie to You
Resolution doesn’t mean squat until that image is headed for a printer. Because the same file can be measured at any resolution, it’s the pixel dimensions that matter most. With enough pixels, a file can be measured at 300 or 3000 ppi. The magic is in knowing how to change resolution without changing the number of pixels. As long as you uncheck the Resample Image option, you can tweak the resolution until Cylons invade and you won’t alter image quality. Honest!

  • anonymous says:

    An excellent article on a topic that causes far too much confusion.

    For clarification, though, when referring to DPI you are only referring to the resolution of an output device, not the image itself. The common (but incorrect) practice of using the two terms interchangeably is one of the things that adds confusion to this issue. This is suggested in the first paragraph under “Resolution and Pixels Defined, and is not correct.

    Digital images are composed of pixels, never dots, and as such their resolution should only be referred to as PPI never DPI.

    The one point the article makes very well, however, is that the only real measure of digital image resolution is the pixel dimensions; PPI is merely an abstract that makes it easier to determine the resolution at a particular output size.

  • anonymous says:

    I am disappointed whenever I read articles of resolution that commit so heavily to defining dpi as something that an end user is under control of.
    DPI, or dots per inch, refers now to a single dot of a printing device. These dots are clustered in a variety of methods within cells (Adobe postscript) which relate directly to pixels of an image. A pixel can contain more information than a dot. The vast majority of images that users will be working with exist as pixels, not as dots. For traditional print folks, the pixel relates to line screen in terminology. While it really is too complex to describe within the limits of maximum word count here, suffice it to say that we have NEVER required 300 pixels per inch for high quality printing of images, but that figure was high enough to ensure that users would not supply images in too low a resolution for print. Whether or not the image is to be manipulated also comes into play as far as the working resolution, which can then still be reduced for the final print file.
    Attempting to explain it in the form used for this article does more damage to the concepts than it helps.
    The truth about resolution needs to address the basic arithmetic before gettting into the math of it – please make sure you get the terminology straight before moving on to the rest of the truth.
    Sincerely,
    Donn Tarris
    22 years in imagesetting.

  • anonymous says:

    No, that’s actually not correct either. DPI refers to the resolution of the output device; the measurement you are talking about is lines per inch, or LPI, which refers to the halftone frequency.

    If you are running film or plates at 150 LPI then the image resolution should be 300 PPI. The resolution of the output device is not relevant here; there are imagesetters and platesetters that run at 1200 DPI, 2400 DPI, 2540 DPI, etc. all of which can ouput a 150 LPI halftone, for which a 300 PPI image would be required. The output resolution of the device may affect the quality of the halftone dots, but not the image resolution.

    Contrary to common thought, the actual math is also not exactly double, but roughly the square root of 2, or 1.4 times the linescreen. So the common recommendation of 1.5 to 2 times the linescreen works, and in fact twice the linescreen is a bit more resolution than is necessary for halftoned images.

    Other screening methods, such as stochastic, use entirely different calculations (that I do not know off the top of my head).

  • Jay J Nelson says:

    I’m rating this story “Strongly Agree” just to balance out the “Strongly Disagree” rating from Donn Tarris. I just re-read the story, and I see that Lesa did differentiate between PPI and DPI, and used them correctly in the right places. And it seems to me that she purposely avoided LPI to avoid complicating the matter even further. Instead, she gives recommended PPI for common printing processes (desktop, newspaper and magazine). Believe me, I’d jump on her back if I thought she was misrepresenting the ppi/dpi relationship, because it’s a vital piece of (mis)understanding for designers. But she didn’t — I think this simplified version is fine for designers. (It doesn’t provide enough detailed information for prepress service providers, but that’s not the intention of the story.)

  • anonymous says:

    Here is the quote that is wrong, or at the least poorly worded and misleading:

    “Resolution, when referring to an image, is the number of pixels displayed per unit of printed length. It’s a measurement used in printing and it’s stated in dots per inch (dpi). “

    These two sentences, in particular the second one, imply that DPI is a valid unit of measurement for an image, which it is not. It is a measurement of output resolution only.

    Image resolution is measured in pixels, *never* dots. Never ever ever.

    She did not mention LPI, that was in response to the other comment that was incorrect.

  • urstwile says:

    The 110% method is a complete myth, and I don’t know where it comes from. I’ve compared this method many times to upsizing an image in 1 shot (both with Bicubic Smoother as the interpolation method), and in fact, the effect on the image of the 110% method is much worse than upsizing in 1 shot.

  • anonymous says:

    Is this method superior to just using the “resize image” tool?

  • anonymous says:

    Despite the obvious errors noted in the previous comments, this article has been reposted exactly as it appeared before, without any corrections. Interesting…

  • anonymous says:

    I don’t believe that the author intended to de-mystify the issues about resolution in this single article. It was an introduction. Not even a primer. Entire books have been written on the subject and they all have room for improvement.

    If we really want to get technical, Epson printers now output at 5400 (+/-) dpi. Converted to ‘pixels equivalences, that would result in 18 dpp based on a 300 ppi image as printed on paper.

    So what do we use to explain resolution – pixels, dots per inch, or printer output capability (and would we use dpp or dpi)? A better question would be how do we explain this number jumble to someone who has limited experience or exposure? To that end, the author did a very credible job.

    If the author made any mistake in this article, it was trying to get too much information to the reader in limited space. Trying to describe ‘resolution’ is one thing. Trying to show how it can be put to immediate use is quite another and would have been better left for Part Deux.

    The bottom line is recognize this piece for what it is – an introduction.

  • anonymous says:

    Great Trick the one unchecking resample image option! Thanks!

  • anonymous says:

    IMO the *worst* time to introduce inaccuracies is when someone doesn’t have the knowledge or experience to recognize the error and/or understand the difference. That simply perpetuates them as fact, reinforces bad habits and accelerates the spread of misinformation.

    I agree that the author is trying to cover a large amount of ground in a short article. That is not an excuse to get it wrong, however.

  • anonymous says:

    I’m glad someone has clarified the fact that projected, monitor and web images are not controlled by DPI. It’s pixels only!

    Many people including noted “Experts” think in DPI for projected images.

  • anonymous says:

    the following paragraph from your article is inaccurate:

    How Much Do You Need?
    The resolution necessary for a beautiful print depends on the printing device itself. For instance, consumer inkjets do a nice job at 225 to 250 dpi, while professional service bureaus require 300 dpi and higher for glossy magazines, coffee table books, and the like. For a color advertisement in a newspaper, you need between 150 to 200 dpi. Same thing for a black and white laser printer. However, to know for sure, you’ve got to run some tests. If someone else is printing your project, ask what resolution they want.

    Here, you are confusing DPI with PPI. The numbers you quote above apply to PPI, but not DPI, which would be half to two-thirds the numbers you quoted. As a rule of thumb, I usually consider that if a glossy magazine requires say, 133 to 175 DPI, then my image would need to be 270 to 350 PPI to have the optimal resolution for printing. Desktop printers do not deal in dots like offset, they print in picoliters which are nearly microscopic, and appear as continuous tone. In that case PPI is a more appropriate measurement than DPI.

    Here’s a tip: consult with your brothers over at Printingforless.com before your next article on resolution.

  • Anonymous says:

    This is a great article, and should help a lot of folks understand the connection between pixel resolution and output resolution. A new misconception, however, has been introduced that I haven’t seen before. It doesn’t change the overall correctness of the article, but it may mislead people a little.

    The reason a 600 dpi image displayed on screen may look the same as a 72 dpi image has nothing to do with our eyes or their ability to perceive detail. It has to do with the fact that the 600 dpi image is not actually being displayed! That huge image has been temporarily down-sampled to the 72 dpi display resolution, so the real 600 dpi image is nowhere to be seen. The display simply can’t display 600 dpi in the first place.

  • Anonymous says:

    The reason a 600 dpi image displayed on screen may look the same as a 72 dpi image has nothing to do with our eyes or their ability to perceive detail. It has to do with the fact that the 600 dpi image is not actually being displayed! That huge image has been temporarily down-sampled to the 72 dpi display resolution, so the real 600 dpi image is nowhere to be seen. The display simply can’t display 600 dpi in the first place.

  • Anonymous says:

    This is great. Thank you!

  • Anonymous says:

    Always confused by the numbers, I have mostly, in scannint photos or slides, used PhotoShop to increase my resolution to, say 2500, and reduce the size to .5 width, usually .4, after doing the PS jobs for cropping, contrast, variations, etc. With the Phaser, I see then the teensts 1/2 postage stamp image; enlarge to 7.5 inch hor.and/or 98 vertical; select Color management, and get a good glossy near 8 x 10. The colors are often darker than what the monitor shows, so one to fly in the Brightness/contrast realm by guesswork as to results, depending on the image itself.
    Jascha Kessler [ignoramus, inc].

  • Anonymous says:

    Great article. Most of this I knew intuitively, but now I understand it better.

    Thanks.
    http://www.BobbyMayberry.com

  • Mike Witherell says:

    The 110% upsampling technique was a method we used in past versions of Photoshop. Like back in PSv5 days. But the algorithms are better now, and if you simply must resample upwards, doing it 10% at a time is no longer necessary. It’s an old truth that is no longer true. Do take note of the generous hints in the dialog box that say (smoother) or (sharper) or (automatic). And in agreement with many of the other comments here, the explanation in this article falls short a great deal. An article about PPI versus LPI versus DPI should at least use these terms correctly. Maybe I should write a primer about this for TrainingOnsite.com!

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