I hope this doesn't come off as too anal or nit-picky, but I pretty consistently see a bit of bad info on image sizes floating around various websites. (I guess it' become a bit of a pet peeve of mine. )
Occasionally you'll see file specifications that use "dpi" or "dots per inch". Someone will say they posted a file at 72 dpi, or even some competitions will ask for files at a certain dpi. This is usually accompanied by a size specification in pixels. For instance, "...a file that's 720x500 pixels at 72 dpi."
In cases like that, the dpi that your image file is set to is completely meaningless. A file saved at 72 dpi is identical to one saved at 720, 7200, or 72000000 dpi. Only the pixel dimensions matter. The dpi number itself is just a piece of metadata, just like the date the photo was taken, or the lens used.
Looked at another way, dpi references inches. But a 700x500 pixel image will use 350000 pixels of your screen no matter what size it is - a 13" laptop or 30" LCD. The dpi has no bearing on this - it's always 350000 pixels.
So a file that 3000x2000 pixels at 72 dpi is exactly the same as one that's 3000x2000 at 7200 dpi when displayed.
The only time dpi matters is for software that works in inches - print drivers mainly. And even then, it just gives the print driver a default amount of inches to assume. It can be easily changed when printing to print at whatever size you want, which leaves the file the same.
A couple of cases where dpi might matter :
1) If someone specs an image at a certain dpi AND number of inches, these can be translated to give you the pixel dimensions. For instance, "8 inches x 10 inches at 200 dpi" would mean a file that's 1600x2000 pixels. I think this is what confuses some people, since "8 inches x 10 inches at 200 dpi" makes sense, but "1600x2000 pixels at 200 dpi" doesn't.
2) Some software like Powerpoint might use the dpi when pulling an image into a slide, since a slide in Powerpoint is specified by inches, so that it matches up with a printed size.
Thanks for clearing that up. I actually had that debate with someone when they suggested that posting at 72 dpi would yield better results for the web than a higher dpi set for the same pixel dimensions.
Nope, not anal or picky - I've seen this confuse tons of folks. About the only thing that leaves even MORE folks confused is how inkjet printer LPI relates to DPI...and why after about 1400 LPI any further increase in "printer resolution" is more of a marketing tool/scam than an advance in printer quality...(except for those who like to examine prints with a loupe, magnifying glass, or microscope).
I agree with you David in most cases, but here is another case where it does matter. When you add text DPI makes a difference. For instance in my web compression action I built, the first step is changing the image size to 300 dpi, if I don't do this and a file has a different dpi when I add my watermark the text size is way off.
The way I understand it, DPI is a term that refers to dots per inch of ink on paper. PPI is pixels per inch in a digital image. Strictly speaking, the two are not interchangeable although many people perpetuate the misuse of these terms by using DPI when they really mean PPI.
So you are correct that the PPI doesn't really matter--only the pixel dimensions matter (unless, as others have mentioned, text is involved). DPI should really only be used when discussing the printing of images.
I think you guys answered something for me... just want to be sure I'm getting this. Since you can open an image in ACR (from Photoshop) at a specified "DPI" (which really should be "PPI" I'm gathering), it seems that value really is irrelevant if your output is on-screen, correct? So if I'm printing then I'll want the Image Size box in Photoshop to state a Resolution 300 or 360 (whatever) dpi to get a decent print, otherwise, the Height and Width in pixels is what it is on screen, end of story... correct?
It almost seems like the Image Size form in Photoshop is really two separate sections: the top is the pixel dimensions that specify how big the image will display on your monitor regardless of "DPI", and the bottom section ("Document Size") is strictly related to printing... so 360 will print a great 8x10, or i can specify 150 if my viewers are standing 20 feet away, allowing much bigger print. THAT'S why that bottom section offers a Resolution box and the top section doesn't... I think I'm starting to get this.
Sorry for the rambling, just trying to get a handle on it all. I think David is dead on... thanks for posting this!
To clarify a little bit what Zack mentioned above, dpi (or ppi) can matter when adding text in Photoshop or other programs, but not necessarily. It depends on how your text is measured. If you set your preferences to add text in point measurements, then the dpi will make a difference. Because points are a typographic measurement related to how big the text is on a piece of paper, it is tied to the print resolution. But if your default is to add text in pixel heights, the dpi won't matter.
Paul, it sounds like you are understanding this issue correctly.
David, thank you for posting this. I usually end up going off onto an annoyed rant, trying to explain the same thing to people as a response on their posts. Your post is definitely much friendlier, and more informative, than what I usually write.
It almost seems like the Image Size form in Photoshop is really two separate sections...
That's correct Paul, as long as you realize that the two sections are related and tied together. For instance, if the "Document Size" is set to 8" x 10" at 200 dpi, you have a 1600 x 2000 pixel image. If you up the dpi to 300, there are two possibilities :
1) You allow resampling, which allows it to create the necessary pixels to keep the image at 8" x 10" and make it 300 dpi. So you'd wind up with an image that's 2400x3000 pixels.
2) You don't allow upsampling, and the pixel dimensions stay exactly the same. In this case, you'll wind up with a size of 5.33" x 6.66" (1600/300 x 2000/300).
Shouldn't you be using the term ppi (pixels-per-inch) here instead of dpi? I have always considered dpi to be an output specification meant to control the printer's dot resolution. For instance, when printing on matte paper I choose 1440 dpi as the printer output resolution, and I might go up to 2880 dpi for glossy/luster paper. In both cases, I have 360 ppi as the image file resolution.
That depends Anil. I'm specifically citing places where I've seen "dpi" used. But you're right, that technically, ppi is the right term. I've just seen them used so interchangably, I think most people think of them that way. I've even seen some people reference an "images' dpi" vs. "the printer's internal dpi" or something like that. I guess if I'm going to be anal, I should go the whole way and make sure to use ppi.
Good discussion to bring up periodically. When I first read about this a few years ago it was difficult to understand. It felt counterintuitive to me after having spent years with dpi embedded in my brain because of a newspaper career.
That's correct Paul, as long as you realize that the two sections are related and tied together. For instance, if the "Document Size" is set to 8" x 10" at 200 dpi, you have a 1600 x 2000 pixel image. If you up the dpi to 300, there are two possibilities :
1) You allow resampling, which allows it to create the necessary pixels to keep the image at 8" x 10" and make it 300 dpi. So you'd wind up with an image that's 2400x3000 pixels.
2) You don't allow upsampling, and the pixel dimensions stay exactly the same. In this case, you'll wind up with a size of 5.33" x 6.66" (1600/300 x 2000/300).
A word of caution and something to keep in mind. David, what you wrote here is KEY. Folks do need to understand that YES, ppi(or loosly dpi) in terms of screen resolution does not matter, only dimensions. However, I expect most photographers like to have their images printed.
Since ppi/dpi doesn't matter from a monitor/web point of view, I RECOMMEND that you use your working PRINTER resolution when working your files. In other words, as David says above, if you set your image resolution to 72dpi(for the web lets say) and set your dimensions to 8x10 for printing you've just lost your ability to print the 8x10 at 300dpi. Because without resampling, you're 8x10 will now be something like 2x2.5" or thereabouts.
So because of this, one of the first things I do to an image is set my dpi to 304.8 (for LightJet5000 printer) with no resampling. If I'm resizing for the web at 720pixels, I don't care what the dpi it. But if I'm working the original file for a 16x20 print, I do care what the dpi is, and it's already set. Other printers may have different requirements, but it's usually between 2-300 dpi, so it's easy to downsize the dpi without effecting the image.
Hope that all makes sense. Good topic David
Posted by Peter Barnes on May 12, 2009 at 11:19 PM
OK, I must be over-anal, but the use of dpi (dots per inch) instead of ppi (pixels per inch, which is what is usually meant) just adds to the confusion that so many seem to have about image size and resolution. To me this confusion of measuring units is as sloppy as talking about kilometres when you really mean kilometres per hour (happened to me yesterday in a phone survey about driving habits). A person new to digital image processing is bound to be confused when they see the same term (dpi) being used to describe the dot resolution of a printer (correctly) and the pixels per inch of a printed image (incorrectly).
As for the rest of David's post, I totally agree. Just yesterday (again) I was asked to send images to a television station (they should know) with these specs: "The files must be 72 dpi, and a minimum of 1024 pixels wide, and 576 pixels high". As David puts so well, the dpi number (they mean pixels, NOT dots) is meaningless and confusing.
Adobe don't help - the 'Workflow Options' for moving an image from ACR to Photoshop include a setting for ppi (at least they get that bit right) when at that stage, when you are not setting a print or screen size, ppi is totally irrevalent. Rant over, thanks for getting this far.
OK, I'll have my whack at it. And please accept my mea culpa. I've been on the road for days, and driving that long can turn anyone's brain to dogshnit.
I came up in photography in newspapers, where it was all dpi. Dots per inch matter when you print ink on newspaper. But now it's pixels per inch, because we've gone digital. And here's the simple thing I've found. When I want an excellent print from White House Custom Color, which does my printing, I transmit an image at 300 ppi. Yep, pixels per inch. Today, as in the old days, 300 dpi (dots per inch) is considered the standard among the best glossy mags.
So you know what I get from all that?
When it comes to making prints, whether I am making my own prints from my Epson inkjet, whether I am sending the files off to WHCC, or whether they are going to a glossy mag, if I send them at 300 ppi, it all works out just fine. And so my workflow is to save the TIFF files at 300 ppi so I can print them, send them for printing or send them for glossy mag reproduction. And, once saved, I resize to whatever pixel dimensions are required by the Web site I'm posting on and finish the image for Web, saving to Web with a different file name so I don't overwrite my high res TIFF.
I have no doubt there are some ambiguities embedded in my grammar and syntax, and folks might glom onto those, but I hope I'm getting across what I want to.
72 dpi... The magic number. Anyone know where it came from? Is it still relevant today?
This was the screen resolution of early Apple computer monitors back in the early eighties; you know, when they booted up on a pair of 5.25 in floppy disks? If you set your image resolution to 72 dpi it would display in its exact size on your screen. You have not been able to buy a 72 dpi monitor since 1984, so why that number is bantered around so much today as "web" resolution is dumbfounding. Most monitors today display between 102 and 120 dpi. If you want to display your images at close to a specific size, up your resolution (I use 102 dpi) and include a few more pixels to get the dimensions you want the viewer to see on screen.
Kelly, tradition dies hard. You are right about the Apples.
Also, the earliest web browsers (Mosiac Beta and Mosiac come to mind) were not all that smart. If you gave them a file that was sized at 400x600, 42 dpi, it would do everything it could to size it at 42 dpi, with horrible results at times. They would display images with a priority of dpi rather than 400x600 pixels. You needed to be very precise in the HTML to display images and poor code could produce some amusing images.
Fortunately, browsers have come a long way since then. As much as we may malign Microsoft, the popularity and capability of IE has gone far towards making the dpi an anachronism.
My images come into ACR at 240ppi. I don't see how it makes any difference whether it's left at 240 or changed to 300ppi at that point, but thought I'd ask since you all are on the subject of ppi/dpi.
It doesn't matter until you go to print Wayne. I'll throw out another example... Say you take an image that's 3000x2000, and it comes into ACR 240 ppi.
1) If you print it like that, without changing anything, the print driver will print an image that's 12.5" x 8.33".
2) If, instead, you change the ppi to 300, without resampling, the image will print at 10" x 8.66".
3) If you resample the image so that it goes from 240 to 300 ppi, the printed size will stay the same, at 12.5" x 8.33", but the actual pixel dimensions will change to 3750 x 2500.
All of this can be altered in the print driver, of course, so this assumes that you just print it as is, straight out of PS or ACR.
David, If I bring the image into ACR at 240ppi, the native 3008x2000pixel image has a doc size of 12.5x8.3. But if I bring it in at 300ppi that same pixel count image has a doc size of 10x6.6 as you say.
Yet if I crop the image to 18x12 inches, the pixel dims remain the same at 3008x2000, and the doc size remains the same, and also the ppi remains the same as both become 166.6
So is it best to bring it into ACR at one or the other? Making it 300ppi in ACR might help if someone who wants to publish it and demands a 300dpi file, therefore eliminating a need to resample?
Wayne - It makes absolutely no difference whether or not you bring it into ACR at 166 or 300 ppi if the pixel dimensions don't change. It's the same file either way until/unless you resample.
As far as submissions go, some places that I've heard of (never submitted anything myself) will specify that they need a file at at least a certain ppi. But they usually specify a size as well. So it will be something like they need a file of at least 18x12 at at least 300 ppi. I can't imagine they'd specify a ppi without specifying the size in inches. If they did, they'd probably still have an image size in inches in mind, and reject anything that wasn't large enough.
Obviously, any image, no matter how small, could be set to be 300 ppi. But that doesn't make it high quality or acceptable for publication.
Posted by Ron McCombe on November 27, 2009 at 12:36 PM
good informative post, sorts out my problems
many thanks
Ron
Posted by Paul DeCesare on December 17, 2010 at 6:02 PM
OK, I think I got it: a 3000x2000 image is just that, a 3000x2000 image. Given NO resampling, how big the image prints depends on the dpi you set. You have 6 million pixels in that image... spread them out to only 150 in an inch, you get a bigger print. Smoosh them down to 360 per inch, you get a smaller print (but nicer quality).
It's that simple, isn't it...
Posted by David Bostedo on December 18, 2010 at 1:32 PM
Discussion
23 repliesOccasionally you'll see file specifications that use "dpi" or "dots per inch". Someone will say they posted a file at 72 dpi, or even some competitions will ask for files at a certain dpi. This is usually accompanied by a size specification in pixels. For instance, "...a file that's 720x500 pixels at 72 dpi."
In cases like that, the dpi that your image file is set to is completely meaningless. A file saved at 72 dpi is identical to one saved at 720, 7200, or 72000000 dpi. Only the pixel dimensions matter. The dpi number itself is just a piece of metadata, just like the date the photo was taken, or the lens used.
Looked at another way, dpi references inches. But a 700x500 pixel image will use 350000 pixels of your screen no matter what size it is - a 13" laptop or 30" LCD. The dpi has no bearing on this - it's always 350000 pixels.
So a file that 3000x2000 pixels at 72 dpi is exactly the same as one that's 3000x2000 at 7200 dpi when displayed.
The only time dpi matters is for software that works in inches - print drivers mainly. And even then, it just gives the print driver a default amount of inches to assume. It can be easily changed when printing to print at whatever size you want, which leaves the file the same.
A couple of cases where dpi might matter :
1) If someone specs an image at a certain dpi AND number of inches, these can be translated to give you the pixel dimensions. For instance, "8 inches x 10 inches at 200 dpi" would mean a file that's 1600x2000 pixels. I think this is what confuses some people, since "8 inches x 10 inches at 200 dpi" makes sense, but "1600x2000 pixels at 200 dpi" doesn't.
2) Some software like Powerpoint might use the dpi when pulling an image into a slide, since a slide in Powerpoint is specified by inches, so that it matches up with a printed size.
Nope, not anal or picky - I've seen this confuse tons of folks. About the only thing that leaves even MORE folks confused is how inkjet printer LPI relates to DPI...and why after about 1400 LPI any further increase in "printer resolution" is more of a marketing tool/scam than an advance in printer quality...(except for those who like to examine prints with a loupe, magnifying glass, or microscope).
Cheers...
Brad
The way I understand it, DPI is a term that refers to dots per inch of ink on paper. PPI is pixels per inch in a digital image. Strictly speaking, the two are not interchangeable although many people perpetuate the misuse of these terms by using DPI when they really mean PPI.
So you are correct that the PPI doesn't really matter--only the pixel dimensions matter (unless, as others have mentioned, text is involved). DPI should really only be used when discussing the printing of images.
It almost seems like the Image Size form in Photoshop is really two separate sections: the top is the pixel dimensions that specify how big the image will display on your monitor regardless of "DPI", and the bottom section ("Document Size") is strictly related to printing... so 360 will print a great 8x10, or i can specify 150 if my viewers are standing 20 feet away, allowing much bigger print. THAT'S why that bottom section offers a Resolution box and the top section doesn't... I think I'm starting to get this.
Sorry for the rambling, just trying to get a handle on it all. I think David is dead on... thanks for posting this!
Paul, it sounds like you are understanding this issue correctly.
David, thank you for posting this. I usually end up going off onto an annoyed rant, trying to explain the same thing to people as a response on their posts. Your post is definitely much friendlier, and more informative, than what I usually write.
That's correct Paul, as long as you realize that the two sections are related and tied together. For instance, if the "Document Size" is set to 8" x 10" at 200 dpi, you have a 1600 x 2000 pixel image. If you up the dpi to 300, there are two possibilities :
1) You allow resampling, which allows it to create the necessary pixels to keep the image at 8" x 10" and make it 300 dpi. So you'd wind up with an image that's 2400x3000 pixels.
2) You don't allow upsampling, and the pixel dimensions stay exactly the same. In this case, you'll wind up with a size of 5.33" x 6.66" (1600/300 x 2000/300).
Shouldn't you be using the term ppi (pixels-per-inch) here instead of dpi? I have always considered dpi to be an output specification meant to control the printer's dot resolution. For instance, when printing on matte paper I choose 1440 dpi as the printer output resolution, and I might go up to 2880 dpi for glossy/luster paper. In both cases, I have 360 ppi as the image file resolution.
A word of caution and something to keep in mind. David, what you wrote here is KEY. Folks do need to understand that YES, ppi(or loosly dpi) in terms of screen resolution does not matter, only dimensions. However, I expect most photographers like to have their images printed.
Since ppi/dpi doesn't matter from a monitor/web point of view, I RECOMMEND that you use your working PRINTER resolution when working your files. In other words, as David says above, if you set your image resolution to 72dpi(for the web lets say) and set your dimensions to 8x10 for printing you've just lost your ability to print the 8x10 at 300dpi. Because without resampling, you're 8x10 will now be something like 2x2.5" or thereabouts.
So because of this, one of the first things I do to an image is set my dpi to 304.8 (for LightJet5000 printer) with no resampling. If I'm resizing for the web at 720pixels, I don't care what the dpi it. But if I'm working the original file for a 16x20 print, I do care what the dpi is, and it's already set. Other printers may have different requirements, but it's usually between 2-300 dpi, so it's easy to downsize the dpi without effecting the image.
Hope that all makes sense. Good topic David
As for the rest of David's post, I totally agree. Just yesterday (again) I was asked to send images to a television station (they should know) with these specs: "The files must be 72 dpi, and a minimum of 1024 pixels wide, and 576 pixels high". As David puts so well, the dpi number (they mean pixels, NOT dots) is meaningless and confusing.
Adobe don't help - the 'Workflow Options' for moving an image from ACR to Photoshop include a setting for ppi (at least they get that bit right) when at that stage, when you are not setting a print or screen size, ppi is totally irrevalent. Rant over, thanks for getting this far.
I came up in photography in newspapers, where it was all dpi. Dots per inch matter when you print ink on newspaper. But now it's pixels per inch, because we've gone digital. And here's the simple thing I've found. When I want an excellent print from White House Custom Color, which does my printing, I transmit an image at 300 ppi. Yep, pixels per inch. Today, as in the old days, 300 dpi (dots per inch) is considered the standard among the best glossy mags.
So you know what I get from all that?
When it comes to making prints, whether I am making my own prints from my Epson inkjet, whether I am sending the files off to WHCC, or whether they are going to a glossy mag, if I send them at 300 ppi, it all works out just fine. And so my workflow is to save the TIFF files at 300 ppi so I can print them, send them for printing or send them for glossy mag reproduction. And, once saved, I resize to whatever pixel dimensions are required by the Web site I'm posting on and finish the image for Web, saving to Web with a different file name so I don't overwrite my high res TIFF.
I have no doubt there are some ambiguities embedded in my grammar and syntax, and folks might glom onto those, but I hope I'm getting across what I want to.
Cheers, all!
This was the screen resolution of early Apple computer monitors back in the early eighties; you know, when they booted up on a pair of 5.25 in floppy disks? If you set your image resolution to 72 dpi it would display in its exact size on your screen. You have not been able to buy a 72 dpi monitor since 1984, so why that number is bantered around so much today as "web" resolution is dumbfounding. Most monitors today display between 102 and 120 dpi. If you want to display your images at close to a specific size, up your resolution (I use 102 dpi) and include a few more pixels to get the dimensions you want the viewer to see on screen.
Also, the earliest web browsers (Mosiac Beta and Mosiac come to mind) were not all that smart. If you gave them a file that was sized at 400x600, 42 dpi, it would do everything it could to size it at 42 dpi, with horrible results at times. They would display images with a priority of dpi rather than 400x600 pixels. You needed to be very precise in the HTML to display images and poor code could produce some amusing images.
Fortunately, browsers have come a long way since then. As much as we may malign Microsoft, the popularity and capability of IE has gone far towards making the dpi an anachronism.
1) If you print it like that, without changing anything, the print driver will print an image that's 12.5" x 8.33".
2) If, instead, you change the ppi to 300, without resampling, the image will print at 10" x 8.66".
3) If you resample the image so that it goes from 240 to 300 ppi, the printed size will stay the same, at 12.5" x 8.33", but the actual pixel dimensions will change to 3750 x 2500.
All of this can be altered in the print driver, of course, so this assumes that you just print it as is, straight out of PS or ACR.
Yet if I crop the image to 18x12 inches, the pixel dims remain the same at 3008x2000, and the doc size remains the same, and also the ppi remains the same as both become 166.6
So is it best to bring it into ACR at one or the other? Making it 300ppi in ACR might help if someone who wants to publish it and demands a 300dpi file, therefore eliminating a need to resample?
As far as submissions go, some places that I've heard of (never submitted anything myself) will specify that they need a file at at least a certain ppi. But they usually specify a size as well. So it will be something like they need a file of at least 18x12 at at least 300 ppi. I can't imagine they'd specify a ppi without specifying the size in inches. If they did, they'd probably still have an image size in inches in mind, and reject anything that wasn't large enough.
Obviously, any image, no matter how small, could be set to be 300 ppi. But that doesn't make it high quality or acceptable for publication.
many thanks
Ron
It's that simple, isn't it...
Yup