dpi question

Discussion in 'Beyond the Basics' started by tonyadgarcia, Feb 26, 2009.

  1. Dwig

    Dwig TPF Noob!

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    Incorrect.

    PPI and DPI, at least when the terms are used correctly, are very distinct. PPI refers to the number of image elements ("picture element", therefore "pixels") per inch. DPI refers to the number of dots the printer's marking engine (that portion of a printer or imagesetter that actually makes marks on the paper or film). DPI is never the correct term when referring to digital image resolution.

    When you print an image, PPI is still the term that's correct when discussing how many pixels are reproduced in every linear inch on the print. It is a measure of the potential image quality, but only when the image is actually printed. Also, its not the PPI stored as a crib note in the image file, but the effective PPI on the paper after all image scaling is accounted for.

    DPI is a measure, to a degree, of the maximum inherent resolultion of a particular printer. You must also take into consideration the number of dots necessary to reproduce a full range of colors. Inkjet printer must put these dots beside each other while dye-sub printers actually stack them on top of each other so a 300dpi dye-sub produces resolution that requires that an inkjet be capable of 1200-2400dpi to match.


     
  2. henkelphoto

    henkelphoto TPF Noob!

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    While 300 dpi is fine for newsprint (actually, our newsprint will only show resolution equal to 200 dpi), magazines often request 400 dpi for submitted photos. For instance, on their commercial website (not editorial) Getty requires 400 dpi as does Sports Illustrated.

    Jerry
     
  3. dcclark

    dcclark TPF Noob!

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    Keep in mind that, if you're not submitting to Getty or SI (as mentioned above), most photos printed at 200 dpi, or even 150 dpi, look great. The quality of the photography itself -- composition, focus, colors, lines -- matter much more than your dpi.

    Finally, there's no real reason to change the dpi setting in photoshop unless you're submitting the photo to somewhere that requires the dpi to be pre-set. Most printing services don't give a rip about the "dpi" marker in the EXIF data, they care about what your print size is, and calculate the dpi from there.
     
  4. Garbz

    Garbz No longer a newbie, moving up!

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    The last two posters, read the whole thread, it's PPI not DPI!!! Pixel = single point of information. Dot = the thing the printer sprays down to draw a pixel. I think if I throw my printer into draft mode then I can actually get down to 300DPI ;)

    Just a note on printing standards, all photos look great at all PPIs it's just a matter of perspective. Literally. 300PPI is a standard where by someone with 20/20 vision will not be able to see individual pixels when the photo is held at extended arms length. That doesn't mean you can't print a photo at 72PPI. Infact if you take the same photo and print it at 72PPI you will end up with a print large enough that you probably want to stand back 1 or 2 metres to appreciate it.
     
  5. prodigy2k7

    prodigy2k7 No longer a newbie, moving up!

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    PPI and DPI is the same IMO. People use it interchangeably so much these days. We make the definitions, the definitions don't make us, definitions change as we change their use... They are equal.
     
  6. dcclark

    dcclark TPF Noob!

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    Garbz, I understand this perfectly well. However, the most useful way to speak of DPI (or PPI if you prefer -- the terms are used interchangeably in most cases outside of the professional publishing industry) is to speak of the number of dots which appear in a single linear inch of a printed image at a specific size. Nobody says "Here's my photo, I want to print it at 300 dpi, what size can I get?" Instead they say "Here's my photo, I want an 8x10 print, how high of a quality will it print at?" -- which is absolutely correct, because that's the most useful information that the typical photographer will want.

    Of course, I qualified everything here as "... outside of the professional publishing industry", because these technical terms do have different meanings there, as you have said. But for the purposes of this thread, that's not useful. I am a mathematician by training, and I could drive normal people to the brink of insanity by pointing out every small error in terminology which is used in everyday life, on the news, etc. -- but I don't, because it usually doesn't matter in such situations.

    I should say that I'm not trying to sound confrontational here, just being practical given the context of this thread. :)
     
    Last edited: Mar 11, 2009
  7. epp_b

    epp_b No longer a newbie, moving up!

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    Exactly. The lower the print resolution, the larger the photo, the further back you'll be standing to properly appreciate the photo. It's not zero sum, but there is an inversely proportionate relationship between print resolution and the distance you stand from a photo at which you cannot pick out individual print dots.
     
  8. Jeffrey Byrnes

    Jeffrey Byrnes TPF Noob!

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    Time to upgrade!!!! I started out using Photoshop 7 back in 2001!!!
     
  9. Garbz

    Garbz No longer a newbie, moving up!

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    So then I ask this: Just because we are not in the professional printing industry should we sit by and simply further the misuse of the terminology? When someone who is learning about computers points to the computer case and calls it the harddrive do you nod or do you correct them? The same can be said about standard dynamic range images which have been tonemapped incorrectly being referred to as HDR.

    DPI and PPI are two different things with two different meanings, and using them interchangeably is incorrect. Even on a computer display this is true as each pixel is a red green and blue dot so DPI is 3x PPI. Every photographic application I have also uses the correct pixel/inch definition.

    I am all for practicality, but not above correcting (unfortunately) common mistakes in terminology. After all people come to these forums to learn too :). Not trying to start a war, just sharing the knowledge.
     
  10. prodigy2k7

    prodigy2k7 No longer a newbie, moving up!

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    Also, stop thinking of it by its definition, think of it by the words...
    "DOTS per inch" well a pixel is a square dot...

    They should be the same definition these days... Since obviously everyone knows whats you are talking about as theya re always correcting you when you say it wrong.

    "DPI" should be renamed because a dot is a dot :p

    I do agree that people should know the difference, but people KNOW what you are talking about when you say it either way, so whats the problem?

    Edit: I believe the difference is: A pixel in a picture on a computer can be any color, one of millions... A "dot" in DPI is a single color (red green blue black?) or whatever printers use to produce that pixel color... There could be 3 "dots" printed to produce one "pixel"

    is that how it works?
     
    Last edited: Mar 14, 2009

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