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Increasing dpi without losing clarity

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  • 14-01-2009 7:09pm
    #1
    GuanYin
    Closed Accounts Posts: 15,552 ✭✭✭✭


    Hey,

    I have an image that is displaying 72 pixels/inch. I want to get this image to a print quality of 300dpi.

    I've been told that pixels/inch and dpi are roughly equatable, but every time I try and increase the image to 300 pixels/inch I lose clarity, especially on the text in the image.

    Can anyone suggest a method of improving the image without losing text clarity.


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    #2
    p
    Registered Users Posts: 2,119 ✭✭✭p


    You can't.

    You're trying to get information from nowehere. The image only has 72ppi of information, so if you increase it to 300ppi, where does all that other 228ppi data other come from? What happens is that the computer just guesses it. If you can only effectively go from his resolution images down to lower resolutions images. So always work from print quality to web.

    You can get away with going from 250 -> 300 sometimes with photos, but with illustrations/text and other things like that you'll get sever artefacts. There are techniques like going from 72>130>220>300, and sharpening the image slightly at each point, but really at 72 dpi, you're best off just spending the time sourcing a different image or recreating it.


    If you wanna research some more of this, the term is upscaling, and I'd say you'll find more techniques online.

    Hope that helps!



    p.s. one last thing. You can change the DPI of an image from 72 to 300 without chanign the image at all. Just uncheck 'resample image' in photoshop and it'll do that. However, that really just changes information about how an image is printed, there will still be the same number of pixels in there.


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    #3
    GuanYin
    Closed Accounts Posts: 15,552 ✭✭✭✭GuanYin


    Thanks so much for that reply, its really, really helpful.

    Yeah, I think I'll just have to go back and have them find the originals.


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    #4
    Setun
    Registered Users Posts: 3,805 ✭✭✭Setun


    Scan the image in hi-res, and bob's your uncle, you've got a big filesize.


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    #5
    Hank_Scorpio
    Closed Accounts Posts: 552 ✭✭✭Hank_Scorpio


    If you scale the image by 24% it will be 300 dpi.

    DPI comes in two variations - actual and effective.

    If the image is 72 dpi that is actual. If you scale it to 24% it is 300 dpi and that's effective. All you're doing is either pushing or pulling the pixels further together or apart.

    If you reduce the image to 24% in size though it may be too small for what you want to use it for, at this point you may need new images.

    But DPI is also relative to what you're printing. If it's a gausian looking image with very little detail then you can probably get away with printing as low as 150 dpi.

    If the image is high in detail (like detailed painting or something) then 300 dpi is your only friend.

    Truly for an image that will be printed at 150lines per inch you can get away with as low as 225 dpi. If you multiply 1.5 x the lpi you get your resolution for your images - 1.5 x 150 = 225 dpi.

    (1.414xlpi is probably good enough. The square root of 2 is 1.414 and deals with pixels rotated at 45 degrees)


    So there's a lot of thought that needs to go into what resolution you really need. For instance, art books are printed at a finer screen of 175 lpi. So 1.5 x 175 = 262.5 dpi.

    It's a safeguard to have all your images at 300 dpi. But in most cases it's not necessary.


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