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Thread: DPI to PPI conversion

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    DPI to PPI conversion

    i'm submitting some images to a magazine by cd.and the magazine has asked for 2 files one at 300 dpi and the other at 72 dpi.i tried to find some sort of DPI to PPI conversion but i'm not having alot of luck.i know that DPI (dots per inch) refers to the output resolution of a printer.and PPI (pixels per inch) refers to the input resolution of a photograph or image.now after a bit of googling the most i can find is that its not that important when submitting images which is most confusing :\

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    Re: DPI to PPI conversion

    There will be a Resize option in your editing software. eg. in Elements 9 it's Image>Resize>Image Size. Which s/w are you using?

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    Re: DPI to PPI conversion

    lightroom4

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    Re: DPI to PPI conversion

    My LR3 got stuck in the mail and is hopefully here tomorrow but,,,the book I bought says the resize is in the Export panel and that you have a PPI option to set at the bottom in the Post Processing tab.

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    Re: DPI to PPI conversion

    We need to clarify some terms, because they all have multiple meanings. We’ll talk about an image’s “Resolution” value last.

    PPI - Pixels Per Inch. This term usually refers to a monitor’s display resolution. For example, my 30” HP monitor has a resolution of about 101.6 PPI. Also, when an image is printed on paper, PPI can refer to the number of image pixels that were printed for every inch of paper. An image itself has no size. It only has an X number of pixels and a Y number of pixels.

    DPI - Dots Per Inch. This term has multiple definitions. For printers, it is usually the number of physical dots that can be printed on an inch of paper. Confusion arises when trying to relate printer dots to image pixels. Most printers require a matrix of dots to recreate one image pixel. That’s why printer specifications give resolution as two values, such as 4800 x 2400 DPI for Canon printers. Such printers have a resolution that photos are printed at, which is sometimes called the Native resolution or Photo resolution, given in PPI. For Canon it’s 600 PPI and for Epson it’s 720PPI. When printing images from these printers, the native PPI values are the only numbers that are meaningful.

    DPI is also used by commercial printers such as Durst or Noritsu wet printers. These printers have lower resolutions, such as 254 DPI for a Durst Theta 76. In this case, one printer dot relates directly to one image pixel. For these printers, a low DPI value doesn’t indicate a lesser ability to print detailed images. The Durst Theta 76 specs claim detail as good as a 1200 DPI inkjet. I have prints from a Theta 76 and they’re really good. Mpix.com uses Theta 76 printers.

    DPI also refers to scanner resolution. Here, one scanner dot will become one pixel in the resulting image. This is important to remember.

    DPI/PPI/Resolution in Software - All imaging software will display a number related to the image. Photoshop calls it Resolution, other software calls it DPI, and still others call it PPI. Common values are 72, 180, 240, and 300. This value causes much confusion. The correct name for this value is DPI, and it refers to the DPI value used to scan the image. When you scan a document using a typical scanner, the DPI used during the scan will be assigned to the DPI value of the image. This allows the image of the document to be printed in its original size. So if you scan at 300 DPI and print the image at 300 PPI, you will recreate the original document.

    Obviously, this is an important value for document imaging software and associated databases. However, images from cameras are not scanned. The DPI value that software displays is either set by the camera to an arbitrary number, or is simply the default of the software. The DPI value has no meaning for images from a camera.

    When it comes to printing photographs, the DPI value doesn’t come into play for one primary reason...we nearly always print borderless. The X and Y dimensions of the image are divided by the number of inches of the paper, and that’s the resulting PPI of the print. So an image with a resolution of 1600x2000 will print at 200 PPI on 8x10 paper, and 100 PPI on 16x20 paper.

    When a publisher asks for images of a certain resolution, it usually means they have no idea what they're talking about. Best thing to do is to simply change the DPI value to what they want and submit your photo.

    In Lightroom you select the Resolution value when you "Export" an image. It's under the Image Sizing section of the Export dialog. Leave "Resize to Fit" unchecked and simply change the "Resolution" field.
    Bootsy and xpatUSA found this helpful.

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    Re: DPI to PPI conversion

    Quote Originally Posted by Andrew1 View Post
    My LR3 got stuck in the mail and is hopefully here tomorrow but,,,the book I bought says the resize is in the Export panel and that you have a PPI option to set at the bottom in the Post Processing tab.
    i've found that. but i dont think it cant be changed to dpi.well if it can i cant find how to do it
    thanks

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    Re: DPI to PPI conversion

    Magazine guys (aka printers) use DPI in their trade language. As such, they confuse the two terms as much as photographers do. I suspect they are after the 300 for a standard print and the 72 for a web based one to review. To be certain, confirm with them.
    Bootsy and GrumpyDiver found this helpful.

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    Re: DPI to PPI conversion

    Quote Originally Posted by Graystar View Post
    In Lightroom you select the Resolution value when you "Export" an image. It's under the Image Sizing section of the Export dialog. Leave "Resize to Fit" unchecked and simply change the "Resolution" field.
    sorry to be a bit thick. but what would you change the Resolution to ?

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    Re: DPI to PPI conversion

    Quote Originally Posted by Bootsy View Post
    sorry to be a bit thick. but what would you change the Resolution to ?
    You'd change it to 300, then export, then do it again using 72. That will give you the two files that you need.


    Quote Originally Posted by Andrew1 View Post
    Magazine guys (aka printers) use DPI in their trade language. As such, they confuse the two terms as much as photographers do. I suspect they are after the 300 for a standard print and the 72 for a web based one to review. To be certain, confirm with them.
    And the distinction is still equally useless. It's going to be the same file with the same pixels. The only different will be value of a meta tag. The images themselves are exactly the same.

    If they had given a DPI value AND actual dimensions for the image (say, 2" x 3" at 300 DPI) you can now do something. That's that the Photoshop crop tool does. You can give it actual dimensions and a DPI (which they call "Resolution".) You select the area of the image you want and Photoshop performs the work necessary to ensure the image will print at the exact size stated.

    But the OP didn't say that a physical print size or display size was specified, so I assume not.

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    Re: DPI to PPI conversion

    Things can become even more confusing here! Print production, such as in a magazine, is achieved for photographs using halftone screens that are described in terms of lines per inch - the number of lines of holes in a conventional halftone screen that is (was) used to convert continuous tone photographs into those little dots that appear to be continuous tones but are in fact dots of varying size.

    Dots per inch and lines per inch do not coincide, because the dots of a line screen vary in size; so it takes many dots (pixels) per inch of a digital file to make one full (round) dot in a line screen.

    Quoting from this link (I looked at a few to find a good one):

    http://www.designtalkboard.com/tips/dtp/dpi.php

    "The finer the screen, the more detailed the reproduced image. Art books may use halftone screens up to 300lpi, whereas magazines are more likely to use 150 lines per inch. Newspapers and billboard posters are likely to use even lower settings. Newspapers because they are printed on rough, uncoated paper require a courser line screen. Posters and screen printed t-shirts can afford a lower line screen as they are designed to be viewed from a relative distance and so the size of the dots will be less noticeable."

    "For printed reproduction, the amount of dots per inch only becomes relevant when calculating the amount of digital information against the intended output size of the image. For example, if the printed requirement of an image is 300dpi, to be output via a 150 line screen, then an image that is 1500 pixels wide and 800 pixels in depth can be printed at a size of 127mm by 67.73mm. Changing the resolution to 350dpi for a 175 line screen printing job, reduces the acceptable output size to 108.86mm by 58.06mm, however the actual dimensions of the on-screen image remain the same – 1500 pixels by 800 pixels."

    So: the publisher knows the line screen value that it uses to print its magazine. They have told you the resolution they require in DPI. Together, this will determine the size at which they can print your image - as determined by the size of the image (in pixels) itself that you supply.

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    Re: DPI to PPI conversion

    Quote Originally Posted by John Morton View Post
    So: the publisher knows the line screen value that it uses to print its magazine. They have told you the resolution they require in DPI. Together, this will determine the size at which they can print your image - as determined by the size of the image (in pixels) itself that you supply.
    But the OP hasn't indicated that they've given him a print size, and even a 10 MP image can be printed at 8"x10" at 300 PPI. Without a print size, you can't calculate pixel dimensions for resizing or cropping your image. The real mystery, though, is why ask for both 300 DPI and 72 DPI images? Surely they can take a 300 DPI image and resize it themselves. It just sounds like the people don't know what they're actually asking for.

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    Re: DPI to PPI conversion

    OK - put simply ...

    Multiply the physical size that the images need to be (in inches) by 300 (for the first image) and 72 (for the 2nd image) - and make sure that the image has that many pixels in it.

    eg if the image is to be printed 6 inches wide by 4 inches high then it needs to be 1800 pixels wide by 1200 pixels high.

    Forget DPI - that's just the printers getting confused (happens all the time).

    It's one of those areas that confuses everyone more than it should -- conceptually it's very very simple.

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    Re: DPI to PPI conversion

    thanks everyone all very helpful

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