What is dpi?

Discussion in 'Photography Beginners' Forum' started by yiplong, Aug 6, 2010.

  1. yiplong

    yiplong TPF Noob!

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    When converting RAW to JPEG, there is an option to enter the DPI. For my camera the DPI is set to 72 by default. I tried entering an much bigger number like 500, but the resulting JPEGs look the same; they also have the same numbers of pixels, but the one with higher DPI takes up more space on the hard drive.
    So what does having more DPI do?
    edit: I know what the term stands for. What I need to find out is how it effects the pictures.
     
    Last edited: Aug 6, 2010
  2. Stormchase

    Stormchase No longer a newbie, moving up!

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    I know some cases its dots per inch. As In T.V. terms. Not to sure how Its applied to cameras
     
  3. inov8ter

    inov8ter TPF Noob!

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    If you look at both on screen say in Photoshop, if you pull up the ruler, you will notice that the one with 500 will be much larger in size, say maybe 20x30. the 72 dpi will be maybe say 5x7. I am just using those as example sizes. Sorry, but I had to do this…here ya go
    Let me google that for you
     
  4. yiplong

    yiplong TPF Noob!

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    I am pretty sure they do not differ in size.
     
  5. clanthar

    clanthar TPF Noob!

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    Hi,

    [​IMG]

    [​IMG]

    The two photos above both contain exactly 59,400 pixels. They are both being displayed on screen actual size. The top photo is 300 dpi and the bottom photo is 100 dpi. If the top photo were printed the print would be 1 inch wide. If the bottom photo were printed the print would be 3 inches wide. Got it?

    Take Care,
    Joe
     
  6. astroskeptic

    astroskeptic TPF Noob!

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    What you're describing doesn't make a lot of sense to me. First, I don't have any raw to jpeg converters (other than ACR) but I would expect DPI to not have anything to do with such a conversion process. (What I would expect of a converter is to simply convert and leave the resolution unchanged.) The next thing that confuses me is why such a tool would use the term DPI since that is a printer spec, not an image spec (images have pixels not dots ... the latter is used to display the former).

    Even though the function of format conversion has nothing to do with image resampling, I suppose it might make sense to build resampling into a converter. But in this case I would simply expect it to prompt for a target resolution in pixels in each dimension, rather than what you're describing. For what you're describing to make sense to me, it would have to additionally prompt for image dimensions because you need that in conjunction with PPI to compute actual image resolution. Maybe there is some hidden setting that defines the image size, but if that were the case, changing the DPI radically like you describe would have a noticeable effect on the pixel count which is apparently not happening.

    If it is resampling, then it should have other input options as well. For example, when you downsample, the issue of aliasing arises which doesn't arise in the upsampling case. These cases require different approaches in order to optimize the result or you could end up with unwanted effects, like image softening when you downsample with the wrong interpolator. If you're not seeing options like this, then its either not resampling (in which case I don't know what it might be doing) or its not giving you control of the resampling process (that's bad).
     
  7. Petraio Prime

    Petraio Prime TPF Noob!

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    It has to do with the fineness of 'dots' used in printing. The correct term is really "lines per inch".

    LPI Lines per Inch - Measuring Halftone Resolution

    Graphics - Dots Per Inch
     
  8. Dwig

    Dwig TPF Noob!

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    Technically, DPI (dots per inch) is always an incorrect term to use when discussing digital images, despite the fact that many programs use the term. The proper term is PPI (pixels per inch). DPI is only properly used when discussing printers and only when discussing their mechanical "marking engine" (that portion of the printer that marks the paper).

    Changing a file's PPI doesn't change anything in the file except a minor notation in the file's header. The actual pixels are not affected by the change and thus no size change is seen in the image. This note is for applications that have their own concept of a virtual page (page layout programs) or some other virtual inch concept (e.g. the rulers in Photoshop). It can be important to know how many of the files pixels will be mapped to a given inch in the final print so having a program translate the file's width in pixels into a width in inches for some given PPI is convienient. Also, when using page layout programs its convienient for images to import at the size the photographer intended by default. By having a PPI specified in the file's header it is possible for the page layout program to size the picture as intended. This latter reason is the reason the concept was invented.

    The setting you are encountering in the RAW converter is there as a convenience. The original RAW usually doesn't have a PPI defined in its header. The converter is designed to assign one when it creates a bitmap out of the RAW.

    BTW, most of the time that you see images import or open in applications, like Photoshop, with a PPI of 72 it is because the file lacks any PPI specification. The field in the file's header, a database like structure, is merely been left blank by the file's creator. The 72ppi value is the common default that application plug in when they encounter such unspecified files.
     
  9. yiplong

    yiplong TPF Noob!

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    Perfect explanation. Thank you.
     

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