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How to find what the DPI in a Jpeg is!!!!!

Discussion in 'Digital Photography & Imaging' started by JMT74, Aug 17, 2007.

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  1. JMT74

    JMT74 Thread Starter

    Apr 14, 2005
    Hello. A friend of mine is working for a magazine and they have specific instructions on what a Jpeg dpi can be. Here is the instructions. JPG-This is an art file type that can be output in 1 color/grayscale or CMYK (full color). JPG files need to be a minimum of 300dpi, and we’d prefer 600-1,200dpi AT THE SIZE IT WILL PRINT. It is always screened. .jpg files can be changed or corrected ONLY by the original designer of the file.

    Her question is: When she receives a jpg... How does she know if it has enough dpi's? If it has too few.. is there a way the original person can increase the number or can they only shrink it? She sent me this picture in question and I found out it has only 72 dpi's. How do you make it 300 or more dpi to print? Thanks.
  2. ChuckE


    Aug 30, 2004
    The DPI (Dots Per Inch) info is nothing but a single entry (well, actually two, one for horizontal and one for vertical, but usually they are the same value). It does not change the picture at all.

    If you want to use an image editor, such as IrfanView, you can use it to change the DPI setting to whatever number that you want.

    If you are desiring a particular final hardcopy size with a DPI setting to match, just take the number of pixels you have for one dimension (for example horizontal) and divine by the number of inches you want that printout to be.

    For example: if you have a picture with 2048 pixels across, and you want it to print in 5 inches, just divide 2048 by 5, or 409.6. Just use IrfanView to change the DPI to 400 (which is close enough, unless you want something more exact. Change it to 410 if you want something closer. (You can't make it a fraction - like 409.6 - so make it 409 or 410.)

    To use IrfanView to change the DPI, just open the picture, and then press the "i" key. There you will see the "Resolution" fields. Enter the value you want and then press the [Change] button, and Save the picture.
    Rest assured that the DPI setting has changed, which will ONLY IMPACT the printers/software combination that actually use the DPI info.
  3. slipe


    Jun 27, 2000
    A real image editor will give you the relationship without the math and probably make it a little more clear. The screenshots are from Photoshop or Elements, but I would think any real image editor would have a similar display. Paint Shop Pro has a similar display. There are several free image editors available if she doesn’t have one.

    The first screenshot is the image size box from a 7Mp image directly from a digital camera that puts 72 pixels/inch (PPI) as the file size. Notice that with “Resample Image” unchecked you can’t change the pixels. You can use those pixels for any combination of PPI and Document Size without actually changing the image. That particular image at 72 PPI would give a 42.667 X 32 inch print.

    If you change the PPI to 300 (second screenshot) the document size becomes 10.24 X 7.68 inches. That is the largest size you can print that image without going below 300 PPI. Note that you haven’t actually changed the image at all – it still has 3072 X 2304 pixels.

    It gets trickier if you want a specific print size. Most images from small sensor cameras are in a 4:3 width to height ratio and most images from DSLR cameras are 3:2. You can’t change that ratio without distorting the image or cropping the image to the right ratio. I presume with the magazine’s policy of “.jpg files can be changed or corrected ONLY by the original designer of the file” that the images will already be cropped to the right size and ratio. Which means the contributor will know the exact size at which it will be printed. In that case the image editor should show both the document size and the PPI at which that size will print.

    The third screenshot is that same image cropped to 10 X 8. Since one of the dimensions is less than the 10.24 X 7.68 maximum size for 300 PPI the image ends up less than 300 PPI – in this case 288 PPI. To get the image larger you must check “Resample Image” and put 300 (or higher) in the Resolution box. That actually changes the image to a larger size in pixels.

    The magazine must have a fussy legal department to have a restriction in their ability to modify an image. A common error for submission is likely that the image comes in RGB rather than CYMK. It is normal for publishing houses to change that along with the image size if necessary. Fitting things without being able to crop the image must be difficult. But the contributor can easily change the mode and pixel size in an image editor. They can also submit an exact print size cropped to the proper ratio.

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