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The 72 dpi myth

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  #1  
Old 03-08-2005, 05:47 PM
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Oceanwatcher Oceanwatcher is offline
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The 72 dpi myth

Is it just me, or are others too frustrated over even high profile documentation stating that monitors have a dpi of 72?

Monitors do not have any dpi at all. And they do not need it. All they need is a resolution.

Also, dpi does not have anything to say for the filesize.

I am just wondering why people still quote these myths. I teach videoediting, and when people ask me what dpi to use for pictures that they are going to import into the editing application, I usually say 3000 just to make them think a bit.... :-D
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Old 03-08-2005, 10:23 PM
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http://dictionary.reference.com/search?q=resolution

"7: (computer science) the number of pixels per square inch on a computer-generated display; the greater the resolution, the better the picture"

Monitors do have dots (pixels). You can see them. You can measure them, so you can work out the number per inch.

Digital pictures are made up of dots. If the number of dots in a horizontal line in your picture is more than the dots available on your monitor (in a horizontal line), you won't be able to view the whole picture without scaling.

The number of dots in your picture will influence the file size. If you want to print with photographic quality, your digital image needs a minimum of 200 dots per inch, and the file size of a picture scanned at 300 dpi will be a whole lot bigger than when scanned at 200 dpi.
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Old 03-09-2005, 03:46 AM
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Quote:
Originally Posted by BigAl
http://dictionary.reference.com/search?q=resolution

"7: (computer science) the number of pixels per square inch on a computer-generated display; the greater the resolution, the better the picture"

Monitors do have dots (pixels). You can see them. You can measure them, so you can work out the number per inch.

Digital pictures are made up of dots. If the number of dots in a horizontal line in your picture is more than the dots available on your monitor (in a horizontal line), you won't be able to view the whole picture without scaling.

The number of dots in your picture will influence the file size. If you want to print with photographic quality, your digital image needs a minimum of 200 dots per inch, and the file size of a picture scanned at 300 dpi will be a whole lot bigger than when scanned at 200 dpi.
Repeating the myth does not make it more true....

A monitor is capable of displaying different resolutions. That mean that a monitors should have a ton of different resolutions. You could say that a monitor at a given resolution compared to it's size have a dpi. But it is totally wrong. This is something people coming from print is using to get their heads around a problem they have been struggling with for years. There are no dpi on monitors. On the other hand, it can display pixels. Many different sizes. The whole clue is that the dpi setting does not change anything on a computermonitor. It is all about pixels.

I agree that the number of pixels influence the filesize. If you look closely, I did not say anything about that. I said that dpi does not influence filesize. Try this: Take a file, change dpi. Save it. Same size picture, different dpi. Same filesize.

If dpi really was relevant for web, then a 3000 dpi picture would display different than a 72 dpi picture. It does not. The 72 dpi stems from way back when someone got the wonderful idea of measuring a monitor and trying to calculate the dpi. And 72 dpi is only true if your resolution is 1024x768 and your monitor is 14 inch.... Anyone still using that?

Even Adobe knows this. Take a look at the resize dialogbox in Photoshop. The size and dpi choices have a clear border around them. This is only for print.

The pixelnumbers have a border around them. This is what gives the size. This and only this. Printsize does not.

BTW - The dictionary is not totally accurate on the first quote. But at least it talks only about resolution per SQUARE inch. Not dpi. But today everyone is measuring resolution on the total of the screen, not on a square inch. So the dictionary has an addition just a couple of sentences down.

1. <hardware> the maximum number of pixels that can be
displayed on a monitor, expressed as (number of horizontal
pixels) x (number of vertical pixels), i.e., 1024x768. The
ratio of horizontal to vertical resolution is usually 4:3, the
same as that of conventional television sets.

No dpi at all. That refers to the printingprocess. But even here the dictionary is not totally correct. If you check, the ratio of 1024x768 is 4:3. BUT - TV is only 4:3 under one conditions - that the pixeldimentions are different. TV needs longer pixels to achieve the 4:3 ratio. If you only go by the numbers.. Well, do the math yourself PAL is 720x576.
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Old 03-09-2005, 05:16 AM
WilliamD WilliamD is offline
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dpi and ppi

I agree absolutely that web graphics don't need to be 72 dpi (and if we're going to be pedantic it should be ppi), however digital images have inherited a resolution requirement from the print industry (hence dpi is so often used in error). As a result most applications expect an image to have a 'resolution' as part of their content. What happens next is largely down to how the application handles (or ignores) that resolution element.

For example, my e-mail application takes an image's resolution into account: If I save the same image @ 50, 75 & 150 ppi & then try to send all 3 images to someone, they will appear on my computer at 3 different sizes - despite them all having the same number of pixels. Open the same images in PS & they're all the same size - add rulers to each image & of course they're all at different scales.

So, ppi can be important in file handling. I know some people find it annoying that their digital cameras assign 72 ppi to all images - because their layout programs will use these files as gigantic lo-res images!

I seem to recall that the origins of the Mac's 72 ppi monitor 'resolution' had some connection with font size descriptions - anyone with a clear recollection of that?? No idea why Windows used 96 ppi...
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Old 03-09-2005, 06:08 AM
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Why 72.........

I don't see this 72 d(p)pi thing as a myth. In fact there are good solid reasons why it is (was) an agreed upon number, although today it may be rather dated.

Let's get the math right first:
Do you still remember when you had a 14" screen at 600 x 800 - maybe just a vague memory now.
(To you, 'coz I'm looking at one right now.)

14" is a measure of the diagonal, so the height/width dimensions (3, 4, 5 triangle) are 8.4" x 11.2".
Multiplying by 72 d(p)pi we get 605 x 806 - pretty darn close.

So that means that if I have an image that measures 144 x 216 pixels and I specify 72 ppi then Photoshop, or whatever, will tell me the picture is 2" x 3", and if I look at this image at 100% zoom on my 14" (600 x 800) monitor it will measure exactly 2" x 3". Which, I think you'll have to admit, is (or, was) pretty handy.

Homework: What d(p)pi are you using on your monitor? You're probably around 88.

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Old 03-09-2005, 08:14 AM
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Thank you ByRo i really was lost till i read what you wrote!
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Old 03-09-2005, 12:35 PM
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Quote:
Originally Posted by Oceanwatcher
The 72 dpi stems from way back when someone got the wonderful idea of measuring a monitor and trying to calculate the dpi. And 72 dpi is only true if your resolution is 1024x768 and your monitor is 14 inch.
I guess nobody mentioned this blatant mistake just out of pure politeness. It is of course rubbish. I was on my way out the door when I realised I had confused a couple of things I actually should know very well. Sinse the 14 inch is diagonal, the with of the monitor is shorter and it was of course on a 640 x 480 Applemonitor this started. My apologies for that mistake.

Still, all examples mentioned here refers to software that is used for printoutput, or aware of that. But dpi really do not come in very handy if your delivery is to a monitor. You have no control over what the viewer will use as her or his resolution, so you have no idea how big it will be on their monitor. I use 1600x1200. Even on a 15 inch laptop.

It is a bit different on TV. There, picturesize is given, and no matter what your dpi is, it will fill the whole screen.

Dpi is wonderful if it is about going out on print. In fact it is much needed. And it is a good thing that Photoshop and other programs know this. But it is still because of print. And sinse the dpi do not affect filesize, it really makes no difference for web or tv what dpi is used. As far as I know, neither IE, Firefox, Opera or any other browser cares about dpi.
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Old 04-10-2005, 05:25 AM
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GillyH GillyH is offline
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Talking Old Joke

Interior, The Starship Enterprise D Bridge....

Picard : "Mister Worf, scan that ship!"

Worf: "Aye, Captain... 300 dpi?"



*cough*
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