DPI to Pixels ???

Discussion in 'Digital Photography' started by Apple Corps, Nov 7, 2007.

  1. Apple Corps macrumors 68030

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    #1
    I am putting together a Keynote presentation for a client who wants high quality images that will be displayed on a 50" 1920 x 1200 Professional Series flat panel monitor. One of the photographers will be sending me a number of images sized for 1920 x 1200 BUT at 72 dpi "resolution".

    I want to ensure that the full capability of the monitors is utilized. How does 72 dpi (printing density) translate to what a 1920 x 1200 monitor "needs" for its highest performance?

    I hope my question makes sense.........
     
  2. Abstract macrumors Penryn

    Abstract

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  3. gr8tfly macrumors 603

    gr8tfly

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    #3
    It has nothing to do with the image, pixel wise. It just ties the dimension to the resolution (number of pixels).

    For instance, a 1900 x 1200 pixel image that is viewed/printed at 8" x 10" would be 190dpi. Whereas, if it was printed on 16" x 20 ", it would be 95dpi.
     
  4. bocomo macrumors 6502

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    #4
    yep, dpi only matters if you are going to print something out

    if it's displayed on a tv, monitor, whatever then the pixel count is all that matters
     
  5. Apple Corps thread starter macrumors 68030

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    #5
    Hmmmmm - could each of you expand on your explanations please? My camera produces a horizontal image "size" of 3504 with all of its 8.2 megapixels. The monitor is only 1920 horizontal pixels.

    I'm not disagreeing with anything - just not understanding how these three factors relate.

    So..........please tell me more.
     
  6. CanadaRAM macrumors G5

    CanadaRAM

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    #6
    Resolution is a ratio. It's the ratio of the dots that are available to the length of output those dots are printed on. In your case, you can forget entirely about the length, and just work with the pixels.

    You have 1920 pixels wide on the monitor. If you get a file of 1920 pixels wide, it doesn't matter what resolution ratio you call it--- it's still 1920 pixels wide.

    If you get a 3000 pixel wide photo from your camera, just shrink it to 1920 pixels wide in Photoshop and you're good.


    So the resolution thing.... that same image, if you displayed it on a 18 inch wide monitor, would be 1920 / 18 = 106 dots per inch display resolution.

    If you had a good colour printer and printed it on a 10 inch wide piece of paper, it would be 192 dots per inch.

    If you printed it onto a 35mm slide, it would be 1920 / 3.5 = 548 dots per cm.
     
  7. gr8tfly macrumors 603

    gr8tfly

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    #7
    As you've probably noticed, when you view images from the camera, you can see the entire image. The software is scaling it to the number of physical pixels on your screen.

    In Photoshop, you'll notice a percentage in the image window title bar. That is the scale factor Photoshop is using while displaying the image. When you zoom in until is says "100%", you will have a 1:1 mapping of image pixels vs. screen pixels. As you continue to zoom in, each pixel will start looking blocky, since the image pixel is represented by more display pixels. At 200%, each image pixel will be drawn using 4 display pixels, etc.

    Hope this helps.
     
  8. Abstract macrumors Penryn

    Abstract

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    #8
    dpi = dots per inch. It's a printing thing.


    An image is made up of pixels. Some are made up of more pixels than others. I mean, an image could be made up of 1 pixel, or even 4 pixels, but you won't see any details. Imagine if someone wanted to draw you on computer, but could only describe all your physical details using 4 dots. It's impossible. Your image is probably around 3500 x 2300 or something, which is a lot of dots.

    Now, with regards to your display and pixel size, since your display can only show 1920 pixels (horizontally) by 1200 pixels (vertically), the largest image you need to use to fill the screen is one made up of 1920 x 1200 pixels. If the image you use is made up of 960 x 600 pixels (ie: the length and width of your photo are made up with half the number of pixels), the length and width of your image will just take up only a small part of your display. In other words, your image will only take up a bit of space in the middle of your screen. That's not what you want.

    If your image is 1920 x 1200 pixels in size, it'll be big enough to fill your display, which can project 1920 x 1200 pixels on the screen.

    If your image is 3000 x 2000 pixels in size, and your display will only display 1920 x 1200 (which is already very very good), then the image will probably be shrunk down to fit the screen. Your screen can't show 3000x2000 pixels.
     
  9. Apple Corps thread starter macrumors 68030

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    #9
    Thanks for the additional info - it does help. The photographer will provide images sized for 1920 x 1200 - my guess is that will help protect them from someone printing large / full quality prints from this file but will still give me all the image quality the monitors can handle - does that make sense?
     
  10. Lovesong macrumors 65816

    Lovesong

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    #10
    Here's something that might help-

    You said that you have a professional 1920 X 1200 50" display.
    That is the exact resolution of my 24" monitor, and my friend's 23" ACD. Having a picture that has a 1920 X 1200 res, will fill up all three of these screens at full resolution. It's just that the size of the dots will be smallest on the 23", slightly larger on my 24", and bigger still on a 50". This is simply the way the displays are made. DPI has nothing to do with it. If anything it is a physical factor of the displays (though that would be PPI-points per inch), and you have no control over that.
     
  11. ACbc macrumors member

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    #11
    Not to hi jack the thread, but while it's here i have a related question. so DPI only has to do with printing, i get that. When editing RAW in Aperture, and exporting to jpeg, there is an option to change the DPI. if i'm interested in making high quality prints.....does it make a difference what i enter here? and if so, at what point does it not matter anymore?
     
  12. Lovesong macrumors 65816

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    #12
    The DPI that you're talking about exporting is typically about 72ppi, which is the "standard" pixel density for displays. Most displays have between 60 and 130 DPI, depending on the pixel pitch (how small the dots are). The bigger the dots, the less of them there are in an inch.

    Printing is different, in that you would need a higher density in order to produce an acceptable-looking picture. Professional prints are typically at 300 dpi, which means that if you have an image that is 1200 X 1800, you can make a nice 4 X 6 printed. Anything lower, and you're going to start noticing artifacts (when you're really up-close and personal with the print). Larger images are designed to be viewed from a distance, which is why it would be acceptable to print them at a lower resolution (200 or below).

    The export on Aperture is made so that the image will appear to have a certain size in inches, so when you're importing in into a document, it will fit within that perceived size. I try not to think about DPI or PPI when I'm dealing with images, but instead think about how large my image is in terms of pixels. If I have a 4300 by 2900 image from my 5D, and I want to print it as large as it goes at full res, I know that I can make a ~9 X 14 (at 300 dpi). Anything larger, would require that I drop my resolution, but if I'm staring at a 30" poster, I won't be doing so with a magnifying glass.

    Is that as clear as mud?
     
  13. ChrisA macrumors G4

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    #13
    The "72" is just a number in the file header. In this application it will by completely ignored. Set it to 1000 if you like. It will make no difference

    What that number does is tell the program that is printing how large to make the
    print. It says "count the dots divide by 72 and make it that many inches."
    In your case this will be ignored and the image will be what ever size the monitor is.

    The key here is that the "72" is ignored and means nothing. Don't worry about it
     
  14. ACbc macrumors member

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    #14
    Clear as mud sounds about right. :)

    So if i have a 1200x1800 pixel image, but i export it at 72 DPI, does that mean the quality of a 4x6 print wouldn't be as good as if i exported it at 300 DPI?
     
  15. ChrisA macrumors G4

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    #15
    Wrong question. You can NOT specify all three of these
    1. Number of pixels
    2. size of print
    3. DPI
    If you specify any two then the other can be computed. Lets go back to the car analogy. Lets say I want a car that uses 4 gallons of gas to go 100 miles and I want 30 MPG. Can't happen. I've over specified the problems same with trying to find someone who is under 30 and born before 1970. Can't happen.

    pixels per inch is pixels divided by inches. If you have 3000 pixels and print them 300 per inch you get 10 inches. No more no less. So if you have 1800 pixels and print tham at 72 per inch you get 1800/72 inches or a really poor quality (maybe "poster quality") 25 inch print.

    But what happens if you DO ask the software to print at 300 DPI and also specify the physical size of the print. The software changes the number of pixels such that 300 x print size = number of pixels
     
  16. ACbc macrumors member

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    #16

    Well, according to aperture's image export preferences it seems as if you can choose all three. if you go to your image export preferences you can choose a size (in my case i keep it at the original size), but right below that, there is an option to enter DPI, which defaults to 72.

    So my question remains, If i export, original size and 72 DPI and I export original size and 300 DPI, will there be a difference print quality from the 2 files?
     
  17. decksnap macrumors 68040

    decksnap

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    #17
    They will just have different print dimensions. If you have the same amount of pixels, the higher the DPI, the smaller the printout. That's all it means.

    Open up an image in photoshop at 72 dpi. Then in the image size box, change the dpi to 300. Then punch the original pixel size back in. you'll see that the image does not change in size or quality at all, just the rulers, because you've changed the amount of pixels it will print per square inch, not the amount of pixels.
     
  18. gr8tfly macrumors 603

    gr8tfly

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    #18
    Only if "resample" is not checked. I've lost track of how many times I've helped someone with images and they haven't noticed the resample was on when they changed the image size for a smaller print. If it resamples going to 300, it will artificially add pixels. Then, going back, it will throw out those pixels. You won't end up with the same image (though, close).
     
  19. Abstract macrumors Penryn

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    #19
    ChrisA is still correct in what he said.


    To me, DPI is fairly useless because I always specify the size of my print by physical size (eg: 8"x10", 8"x12", 10"x10"). Lets say you have a 6 MP image that's 3000x2000 pixels. You want to print an 10" x 8" photo (where 10" corresponds to 3000 pixels). If your image has 3000 pixels along its length, and you want your print to be 10 inches long, it means you want the printer to print 300 dots per inch. If you specify the number of pixels in your image, and the physical size you want to print at, there's no other way for the printer to interpret this information.
    You don't need to specify the DPI anymore. You already did that when you told the printer to print a 10" photo using 3000 pixels.

    If you said you wanted to print a photo that has 3000 pixels one one side, and you want it to be printed to 10 inches, and then using Photoshop, you specify a dpi of 50, it's impossible to print at 50 dpi.

    If you have a photo with 3000 pixels on one side, and you specify the DPI to be 100 dpi, then you don't need to specify the physical size of your print (eg: you don't need to say "I want an 8x10" photo).

    You only need to give the printer 2 parameters.
     
  20. decksnap macrumors 68040

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    #20
    Doesn't matter the way I described.. because you punch it all in in the same dialogue.
     
  21. wmealer macrumors regular

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    #21
    Jeez, guys! The OP's question was answered already. You guys are making this much more complicated than it has to be. Feed a 1920 x 1200 display a 1920 x 1200 image, and the "full capability of the monitor" will be utilized. The dpi plays no role in this problem whatsoever.

    I understand the propensity for designers to wax poetic about all things technical, as I have been known to do myself. But it's totally unnecessary here. Just answer the question and move on, for Pete's sake! :p
     
  22. gr8tfly macrumors 603

    gr8tfly

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    #22
    If "resample" is OFF, you won't be able to independently set all three, because, as you say, they are tied together. You can't change the number of pixels, for instance. If it's ON, you can change any of them, and it will create or remove the pixels as necessary for the DPI or new pixel count (or, if you change inches, and leave DPI the same, it'll change number of pixels).

    Anyway, it's a different game if resample is ON. It's just a word of caution, I guess, because so many times someone has resample on, takes their 3000 pixel image, leaves the DPI alone and changes the dimensions. It resamples the image down to maintain the same DPI at the smaller dimension.

    Example: 3000 pixels at 10" = 300DPI. Now, they tell it to make it 5" wide, but have resample ON. The new DPI will still be 300DPI - but, now they have 1500 pixels instead of the original 3000. (Worse yet: They hit save, and, of course, it's their original they're working on and they've now lost 50% of their resolution. It actually does happen.)

    The difference in print quality at 300DPI vs 600DPI probably won't be noticed by anyone. But, say you start at 150DPI and reduce the image size thinking you'll get 300DPI. Resample kills that, because it will throw out pixels to maintain 150DPI.

    When using Image Size (Photoshop, or elsewhere), it's a good "safety tip" to double-check the setting of "resample" - generally, it shouldn't be ON, unless the file size needs to be reduced.
     
  23. gr8tfly macrumors 603

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    #23
    The OP asked for clarification on some points. You could, maybe, not read them?
     
  24. ACbc macrumors member

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    #24

    Ok....so the plain english answer seems to be, specifying the DPI when exporting to JPEG doesn't matter. :D

    Thanks abstract and chris for explaining that in great detail. :)
     
  25. Apple Corps thread starter macrumors 68030

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    #25
    The scary part is I think I understand it - certainly better than before!! Thanks for all of your input.

    Now for a related question - if 99.9% of my photographic work is going for HD displays (as opposed to prints) do I need to worry much about the megapixel race (cameras that produce 3500+ horizontal pixels)? Many other features come into play I know - so this question is just about the megapixel and resolution thing.

    Thanks again.....:eek::eek::eek:
     

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