Not Your Everyday Banana

by Bart Arondson

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I often see statements in discussions about web displayed image quality that go something like this: "I only upload pictures at 72 dpi and 1200 pixels on the long edge, so if someone copies and prints the image it won't look as good as if it were 300 dpi."

What? Have I missed something here?

Does an image edited and saved/exported with 1200x800 pixels at 300 ppi look any different online than the same image saved/exported with 1200x800 pixels at 72 ppi? Other than the metadata regarding ppi, is there any difference in the two images at all? If so, what is the difference? If I tell my printer to print the images at 4R (4X6 inches) will there be any difference at all in how the printer creates a print from the 1200x800 @ 300 dpi image versus the 1200x800 @ 72 dpi image?

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5 Answers 5

up vote 35 down vote accepted

You ask if there is a practical difference. So the answer is yes, albeit a very small one, but some of the other answers have missed it.

You're right that the only difference is in the metadata: if you save the same image as 300dpi and 72dpi the pixels are exactly the same, only the EXIF data embedded in the image file is different. (I've even verified this using a Beyond Compare, a file comparison tool.) If you open the two images on screen you will see absolutely no difference between them.

However, now drag and drop those two images into a word processor and you should see something like this:

enter image description here

Page-setting software like InDesign does the same thing. This is because in both cases the target environment is one that measures things in real-world units (centimetres or inches), so it uses the dpi metadata to decide how to convert your image's pixel dimensions to real-world dimensions. For example, a 600x600-pixel image at 300dpi will appear on the page at 2x2 inches.

By contrast, most screen-based environments (Photoshop, the web, etc.) measure things in pixels so no conversion is needed: each pixel in your image simply occupies one pixel of your screen.

So, if you're preparing an image for print on paper or other physical media and you're asked for a specific dpi (which will usually be 300), you should stick to it to ease the workflow at the print end. (Of course, a page designer can always convert your 72dpi image to 300dpi without losing anything, but why make things difficult?) Note that this only ever applies if your image is going to be placed on a page (for example, in a magazine or book), which is why it so rarely makes a difference. If you're just printing photos full-page (either on your own printer or sending off for photographic prints) the dpi will make no difference.

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It seems like if this were really the goal (in the context of the question!), the "best" thing to do would be to use 1dpi, or 1,000,000. –  mattdm Jan 14 at 12:08
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The answer opens with an introduction that says the gist of the entire answer: there is one small practical difference. It then states where there is no difference: in the pixels themselves and how they are displayed by most photo applications on a monitor. Only then does it move on to discuss the situation where ppi does matter: in word processors and desktop publishing applications. The illustration shows how the ppi affects the size of a photo within another document. The answer then restates that most screen-based environments measure in pixels only. –  Michael Clark Jan 14 at 12:44
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There is then a nice summary to end the answer: If the image is placed in a page it matters, if you are just printing full-page photos it doesn't. –  Michael Clark Jan 14 at 12:45
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Those who "...just read... quickly without really understanding..." will often get things wrong. There's not much you can do about that. I guess we could emphasize certain parts of the answer in bold font. –  Michael Clark Jan 14 at 12:48
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Matt: You're absolutely right, "it doesn't really matter". Nowhere do I say that it does. It's a minor issue of convenience for the person setting the page who asked you for a 6x4" image at 300dpi, that's all. I've tried to explain that as clearly and factually as possible. Any answer will be misread by a person who skims it, but the number of upvotes indicates plenty of people have found it helpful. Michael: Thanks for your comments. –  Mark Whitaker Jan 14 at 14:54
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Does an image edited and saved/exported with 1200x800 pixels at 300 ppi look any different online than the same image saved/exported with 1200x800 pixels at 72 ppi?

No.

A bitmap produced either on-screen or on paper from the image will be identical.

The only difference would be the default print size from some applications, and only then if the image size is not specified in any other way.

If you were to open the image in a simple image viewer (something akin to MS Paint that handles images, but not page layout, such as Adobe Illustrator) and press Print, you may find it sets the default print size based on the resolution, so the 300 PPI print would be 6x4" while the 72 PPI print would be 17x11"... assuming it didn't just auto-fit to the default paper size.

The only way to stop someone printing it at higher than 72 PPI on a 6x4" photo would be to only upload the image at 432x288 resolution.

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As you wrote it, the answer is that there is no difference (until you print it or look at it in a document that will be printed).

First a clarification: PPI is pixels per inch, a description of the resolution of the image. DPI is dots per inch, a description of the physical ability of the printer/scanner being used.

pixels (on a side) = ppi x inches. multiply the two sides to get the total size of the picture, usually measured in MP (megapixels).

My local paper runs a section where people can submit their photos, and the instructions are that the pictures "must be at least 300 PPI". I've always been tempted to turn in a 300x300 pixel image at 300 PPI. This of course is low-res (.09 MP) and only prints at 1"x1", but would meet their posted requirements.

The assumption behind statements like this is that the image is a "reasonable size" when printed (like 4"x6" for example). In that case, 300 PPI means it would also look sharp (contain a lot of pixels, in this case 1200x1800=2.2MP), compared to 72 PPI which would look, well, pixelated at .1MP.

In your example they are measuring by pixels ("1200 pixels on the long edge"), not in inches, so then the PPI value is immaterial.

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A careful reading of the question will show that ppi is used throughout, other than when quoting how others (mis)state it. The ppi field in the EXIF apparently must be populated with some number when the image file comes out of the camera to be compliant with the DCF standard. –  Michael Clark Jan 8 at 18:21
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There's no difference. The statements you are seeing are uninformed.

There are a lot of uninformed people on the internet, so that's not surprising. You haven't missed anything, except maybe you are overestimating the reasonableness of typical comments you might find online. :)

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@MichaelKjörling *Someone (yes, I had to make a comment for that, someone was wrong on the Internet :) –  heinrich5991 Jan 9 at 9:16
    
@heinrich5991 Aw, drat. I never thought it would be me! ;) –  Michael Kjörling Jan 9 at 9:23
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I suppose that it depends on who is doing the speaking.

If the speaker has half a clue, you should understand their statement to mean that they're saving the image at the same dimensions but lower resolution. That is, if your goal is to provide a preview image that can't be used to make high quality prints, you save it as (for example) an 8"x10" at 72dpi instead of as an 8"x10" at 300dpi.

If the image that the speaker is saving turns out to be 24"x40" at 72dpi instead of 8"x10" at 300 dpi, you can assume the speaker has a fundamental misunderstanding of how images work.

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What the question says is that the images are exported/saved at 1200x800 pixels (a 3:2 aspect ratio, by the way, not 5:4) with a ppi setting of 72 ppi or 300 ppi respectively. If printed at those dimensions obviously the 300 ppi version would be 4 x 2 2/3" and the 72 ppi version would be 16 2/3 x 11 1/9". But any remotely modern printing application will print a 1200x800 pixel digital image on 4R paper by resizing it exactly the same way. –  Michael Clark Jan 8 at 7:05
    
@MichaelClark No argument. The OP has since edited the Q to include "and 1200 pixels on the long edge" in the alleged statement. That clarifies things somewhat and shifts the focus to what you're talking about. –  Caleb Jan 8 at 7:48
    
@MichaelClark I humbly suggest that your comment here makes the accepted answer wrong. Of course it's true that some applications behave that way, but it has no real implications for image quality, especially when you include this clarification. –  mattdm Jan 14 at 12:04
    
It's not an either/or Matt, it is both. The question asks in general if there is any practical difference between a 1200x800 pixel image saved with a ppi of 72 and a 1200x800 pixel image saved with a ppi of 300. One (of several) sub-questions asks if a 4R print will look any different. Another sub-question asks if there is any difference and if so, what? –  Michael Clark Jan 14 at 12:27
    
The accepted answer addresses both of those sub-questions. This answer addresses neither, and in fact misstates the question to say the image is being saved at different, rather than identical resolutions. –  Michael Clark Jan 14 at 12:29
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