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A technical question...

  • 25-07-2010 9:17pm
    #1
    Ricky91t
    Closed Accounts Posts: 6,281 ✭✭✭


    Hello all,

    I've had this question niggling at the back of my head for quite some time now, And just as I forgot it, Chorcai mentioned that submitted photos for the 24 hours magazine should be 150DPI...

    So, I googled my question to no avail, so here it is.

    When I shoot a JPEG file on a camera what DPI is it?
    When I shoot a raw file on a camera what DPI is it?
    If it's say, 70DPI then surely changing it to 150DPI or 300DPI is kind of wrong as you're attempting to cram more information into each inch...

    Can anyone answer this for me?

    Thanks


Comments

  • #2
    AnCatDubh
    Registered Users, Registered Users 2 Posts: 6,393 ✭✭✭AnCatDubh


    dpi has no effect in terms of your camera. You are dealing with a sensor size which yields height x width pixels - while most cameras will allow you select smaller or larger sizes, you are selecting different heights x widths.

    a jpg or raw should yield identical height x width of an image. raw will simply have more information - the stuff the camera disposes of when it processes the raw sensor data to a jpg format.

    If you have an image from a sensor for example 3000 height x 3000 width (keep it simple) and your printer is printing at 300dpi then the resulting image will be 10 x 10 inches. If your printer is printing at 150dpi then you end up with a possible 20 x 20 inches print.

    The dpi is just a measurement of how much print inches would you get from an height x width images when printed at a particular setting (dpi).

    I think that's the gist of it.


  • #3
    Chorcai
    Registered Users, Registered Users 2 Posts: 2,283 ✭✭✭Chorcai


    This might help http://en.wikipedia.org/wiki/Dots_per_inch

    DPI refers to the physical dot density of an image when it is reproduced as a real physical entity, for example printed onto paper, or displayed on a monitor. A digitally stored image has no inherent physical dimensions, measured in inches or centimetres. Some digital file formats record a DPI value, or more commonly a PPI (pixels per inch) value, which is to be used when printing the image. This number lets the printer know the intended size of the image, or in the case of scanned images, the size of the original scanned object. For example, a bitmap image may measure 1,000 × 1,000 pixels, a resolution of one megapixel. If it is labeled as 250 PPI, that is an instruction to the printer to print it at a size of 4 × 4 inches. Changing the PPI to 100 in an image editing program would tell the printer to print it at a size of 10×10 inches. However, changing the PPI value would not change the size of the image in pixels which would still be 1,000 × 1,000. An image may also be resampled to change the number of pixels and therefore the size or resolution of the image, but this is quite different from simply setting a new PPI for the file.


    For vector images, there is no equivalent of resampling an image when it is resized, and there is no PPI in the file because it is resolution independent (prints equally well at all sizes). However there is still a target printing size. Some image formats, such as Photoshop format, can contain both bitmap and vector data in the same file. Adjusting the PPI in a Photoshop file will change the intended printing size of the bitmap portion of the data and also change the intended printing size of the vector data to match. This way the vector and bitmap data maintain a consistent size relationship when the target printing size is changed. Text stored as outline fonts in bitmap image formats is handled in the same way. Other formats, such as PDF, are primarily vector formats which can have bitmaps pasted into them. In these formats the target PPI of the bitmaps is adjusted to match when the target print size of the file is changed. This is the converse of how it works in a primarily bitmap format like Photoshop, but has exactly the same result of maintaining the relationship between the vector and bitmap portions of the data.




    http://www.dpiphoto.eu/dpi.htm

    I own a 4 MP camera.
    When I’m taking pictures, ratio 4:3, the picture will be 2304 x 1728 pixels, and W x H in pixels is then 3.981.312 = (rounded off upwards) 4 MP.
    In ratio 3:2 they will be 2304 x 1536 = 3.538.944 = (rounded off) 3,5 MP.


    _______


    The reason I was on about 150dpi that is what I had to downsample the images to 150dpi, and these then got downsampled again to 100dpi by issuu.


    Thankfully no one sent in any images at 150dpi ( 99%sure) this means the magazine produced, with a few extra bits like bleeds/gutter adjustments it is print ready.


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