Pixels vs dpi

Discussion in 'Photography Beginners' Forum' started by Dieselrider, Feb 15, 2008.

  1. Dieselrider

    Dieselrider TPF Noob!

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    I am a beginner in the digital area (I never was real experienced in film for that matter) so, please bare with me if my questions seem fairly elementary. I am using a canon digital rebel xti 400D and having a blast with it so far. I was using a little samsung point and shoot before but, it took forever to be ready for the next picture to be taken. According to the camera and information settings on the rebel, I am taking pictures in the 10 megapixel range. How does that translate to dots per inch on a computer?
    If I ever get to the point I can take photos I feel are good enough to sell, I would like to know more of how to format them to the requirement the publishing houses would want. They seem to be expecting 300 dpi or better for most editorial stuff. How does that translate into megapixels or, are there no relations between the two? Thank you for your time.:thumbup:
     
  2. Dieselrider

    Dieselrider TPF Noob!

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    I think I may have found the answer here. http://www.design215.com/index.php

    But, what about when someone want a low resolution image submitted for review, how do you convert your high resolution photographs from high resolution to low resolution?
    I am also seeing that some publishers are requiring high resolution in the 50MB range. What's up with that? At highest quality settings my camera takes it gives me about a 3-4MB size file. Does Adobe or other programs allow you to resize them to these specs? I am definitely over my head here.
     
  3. Alpha

    Alpha Troll Extraordinaire

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    DPI is a matter of print resolution. End of story. Neither your camera nor your monitor see in "dots." The appropriate term for the image on your computer is PPI (pixels per inch). DPI is a misnomer that technically refers to the print resolution of the image. They can be used somewhat interchangeably if you operate under the assumption that one pixel will print as one dot.
     
  4. Alpha

    Alpha Troll Extraordinaire

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    No, you will not be able to increase your resolution form 3-4mb to 50mb. If they're asking for a very high resolution file, they want 1:1. Upsizing the image to create a larger file would not increase the resolution. You would essentially be turning every pixel into more pixels of the same color.
     
  5. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Welcome to the forum.

    You can use software (Photoshop etc) to change the size & resolution of your image. Downsizing is fairly easy and you can upsize, but since the software has to 'make up' the extra pixels...the quality will degrade as you get bigger.

    To downsize your images, you can do two basic things. The first is to reduce the size. Your camera can record images at 3888 x 2592 pixels. For web viewing, you could reduce that to 800 x 533 give or take. The next step is Jpeg compression. When you save a Jpeg, you can choose to use more or less compression/quality.
    Those two steps can significantly reduce the file size of an image, which will allow for easy up/down loading or E-mailing etc.
     
  6. EOS_JD

    EOS_JD TPF Noob!

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    Learn to read your image in pixel size rather than cm and inches and it'll make more sense

    Web sized images are (as Mike says) around 800x600 pixel size. (sometimes smaller)

    This is the formula I use to work out any unknown from my digital image

    Px = Pixels
    R = Resolution (ppi)
    Ps = Print Size

    If you want to know how big you can print at say 300ppi with your high res image the formula is

    Ps = Px/R

    2 axis so s calculations
    Ps = 3888/300 = 12.96"
    x
    Ps = 2592/300 = 8.64"

    So almost a 12.96 x 8.64 image at 300ppi. If you wanted to print bigger, your resolution would need to be smaller. Lets say you want a 21" x 14" image (same 3/2 ratio). the resolution that this would print at would be:

    R = 3888/21 = 185.14
    R = 2592/14 = 185.14

    This is getting low however in a big print like this, 185ppi should be fine.

    Hope you see the relationship using that equation. Once you have tried it a few times and it becomes clear, it will change the way you look at images.
     

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