Scale, dimensions, and resolution

Discussion in 'Digital Photography' started by Phil Stripling, Jul 18, 2003.

  1. I'm completely confused by these concepts. Say my camera takes photos that
    are 640 x 480 pixels (about 8.6 x 6.7 inches) wide and high. I open the
    image using a software program, and it tells me that the image has a size
    of 640 x480 and a resolution of 72 ppi.

    Let's say I want to print this on an 8 x 10 inch sheet of paper. I can use
    my software to 'scale' the image from about 8.9 x 6.7 inches to about 10 x
    7.5 inches (the closest I can get to 8 x 10 with a 480 x 640 image), but
    the 72 ppi remains the same.

    Camera manuals refer to the dimensions (640 x 480) as a 'resolution,' and
    my software says that's the dimensions. It says that 72 ppi is the
    resolution. My printer says it prints at a resolutions of 300, 600, or 1200
    dpi.

    Scaling the picture up from 640 x 480 to 720 x 540 still leaves me with a
    72 ppi resolution. Some software will let me increase the 72 ppi to 144 or
    300 by interpolation or fractals, whatever. This affects the dimensions of
    the image.

    So what's a good resource to study up on this? I want to figure out how to
    get semi-decent prints from digital images.
    --
    Philip Stripling | email to the replyto address is presumed
    Legal Assistance on the Web | spam and read later. email to philip@
    http://www.PhilipStripling.com/ | my domain is read daily.
    Phil Stripling, Jul 18, 2003
    #1
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  2. Phil Stripling

    Guest

    Phil Stripling <> wrote:
    : I'm completely confused by these concepts. Say my camera takes photos that
    : are 640 x 480 pixels (about 8.6 x 6.7 inches) wide and high. I open the
    : image using a software program, and it tells me that the image has a size
    : of 640 x480 and a resolution of 72 ppi.

    : Let's say I want to print this on an 8 x 10 inch sheet of paper. I can use
    : my software to 'scale' the image from about 8.9 x 6.7 inches to about 10 x
    : 7.5 inches (the closest I can get to 8 x 10 with a 480 x 640 image), but
    : the 72 ppi remains the same.

    : Camera manuals refer to the dimensions (640 x 480) as a 'resolution,' and
    : my software says that's the dimensions. It says that 72 ppi is the
    : resolution. My printer says it prints at a resolutions of 300, 600, or 1200
    : dpi.

    : Scaling the picture up from 640 x 480 to 720 x 540 still leaves me with a
    : 72 ppi resolution. Some software will let me increase the 72 ppi to 144 or
    : 300 by interpolation or fractals, whatever. This affects the dimensions of
    : the image.

    : So what's a good resource to study up on this? I want to figure out how to
    : get semi-decent prints from digital images.
    : --
    : Philip Stripling | email to the replyto address is presumed
    : Legal Assistance on the Web | spam and read later. email to philip@
    : http://www.PhilipStripling.com/ | my domain is read daily.

    You might want to take a look at www.scantips.com

    Ray

    --
    E. Ray Lemar
    , Jul 18, 2003
    #2
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  3. Phil Stripling

    Jim Townsend Guest

    Phil Stripling wrote:

    > I'm completely confused by these concepts. Say my camera takes photos that
    > are 640 x 480 pixels (about 8.6 x 6.7 inches) wide and high. I open the
    > image using a software program, and it tells me that the image has a size
    > of 640 x480 and a resolution of 72 ppi.


    OK.. It isn't that hard.

    First off you have to realize the term INCHES means inches of paper. The
    'inches' in dots per inch and pixels per inch both refer to inches of paper.

    PPI is a printing term. PPI resolution refers to the PRINTING resolution. If
    your software shows an image is 5 x 7, then that's how big it is going to be
    on paper. It has no other meaning.

    Once you understand that the rest is easy.

    The size of an image depends on two things and two things only. The total
    number of pixels and how many pixels you spread across each inch of paper.

    Take a 1000 pixel wide image. Spread the pixels out at 100 for each inch. The
    image will be 10 inches wide. 1000 pixels divided by 100 pixels per inch = 10
    inches. It's very simple math.

    So a 1000 pixel wide image printed at 100 ppi will be 10 inches.

    The more pixels per inch you have, the better the image will look. But you're
    stuck with how many pixels you have. In the above example, a 1000 pixel wide
    image can only give you 100 pixels per inch when printed at 10 inches.

    To print a 10 inch image at 300 pixels per inch, you would HAVE to have a 3000
    pixel wide image. 3000 pixels / 300 ppi = 10 inches.

    If you *needed* to print your 1000 pixel wide image at 10 inches at 300 pixels
    per inch, you would could get by by resampling it and making it into a 3000
    pixel image. Now it will work. The only thing is, you aren't getting any
    more details in your image.

    Without changing the number of pixels in the image, it's mathematically
    impossible to print a 1000 pixel wide image 10 inches wide at 300 pixels per
    inch.

    See the relationship ?

    As far as your software saying 72 dpi.. And perhaps your camera showing
    150ppi.... Again, these are printing terms. Your software and camera have
    plugged in default PRINTING values.

    Image files have a couple of bytes in the header where the ppi (or dpi) info is
    stored. These couple of bytes are there to tell your printer how BIG to print
    the image on paper.

    The 72 dpi or 150 dpi (or whatever happens to be placed there) are *default*
    values. It means that the picture will be a certain default size when you
    print it on paper.

    You can *change* these values to whatever you want at the time you print it.
    The DPI and PPI values have no effect on the quality of the image... Just the
    size when it is put on paper.
    Jim Townsend, Jul 18, 2003
    #3
  4. Phil Stripling

    John Navas Guest

    [POSTED TO rec.photo.digital - REPLY ON USENET PLEASE]

    In <> on 18 Jul 2003 11:40:31 -0700, Phil
    Stripling <> wrote:

    >I'm completely confused by these concepts. Say my camera takes photos that
    >are 640 x 480 pixels (about 8.6 x 6.7 inches) wide and high. I open the
    >image using a software program, and it tells me that the image has a size
    >of 640 x480 and a resolution of 72 ppi.
    >
    >Let's say I want to print this on an 8 x 10 inch sheet of paper. I can use
    >my software to 'scale' the image from about 8.9 x 6.7 inches to about 10 x
    >7.5 inches (the closest I can get to 8 x 10 with a 480 x 640 image), but
    >the 72 ppi remains the same.
    >
    >Camera manuals refer to the dimensions (640 x 480) as a 'resolution,' and
    >my software says that's the dimensions. It says that 72 ppi is the
    >resolution. My printer says it prints at a resolutions of 300, 600, or 1200
    >dpi.
    >
    >Scaling the picture up from 640 x 480 to 720 x 540 still leaves me with a
    >72 ppi resolution. Some software will let me increase the 72 ppi to 144 or
    >300 by interpolation or fractals, whatever. This affects the dimensions of
    >the image.
    >
    >So what's a good resource to study up on this? I want to figure out how to
    >get semi-decent prints from digital images.


    It's actually quite simple. You tell your printing software what size to
    print the image, and either the printing software or printer driver will scale
    (resize, resample, upsample) the image to the resolution of the printer. So
    if your image is indeed 640 x 480, and you want to print it as 8.6 x 6.45
    inches (uncropped, same aspect ratio), and your printer is set for a
    resolution of 300 dpi, then the image will be upsampled to 2580 x 1935 for
    printing, a ratio of about 4:1 in each direction. That's a lot of upsampling,
    so don't expect a terribly good print.

    You only need to worry about upsampling manually before printing if you have
    software able to do a noticeably better job of upsampling than the printing
    software or printer driver (e.g., Genuine Fractals).

    Assuming that the image has been taken with a good lens, for printing figure
    at least 130 PPI (pixels per inch) for acceptable results (at normal viewing
    distances), and up to 230 PPI for excellent results. With current technology,
    anything more than 300 PPI is pretty much wasted.

    4x6 5x7 8x10
    ------ ------ ------
    Acceptable: 0.4 MP 0.6 MP 1.4 MP
    Very good: 1 MP 1.4 MP 2.5 MP
    Excellent: 1.4 MP 2 MP 4 MP
    Best: 2.1 MP 3 MP 7 MP

    640x480 = 0.3 MP
    1024x768 = 0.8 MP
    1600x1200 = 1.9 MP

    Note that there is much more to the quality of digicam images than the raw
    pixel count. The quality of the lens is an important item that is often
    overlooked -- I'd usually go for 1.4 MP taken with a high-quality lens over
    2.1 MP taken with an inexpensive consumer lens.

    Note also that the above is only my own opinion, and that opinions can and do
    vary widely. ;-)

    --
    Best regards,
    John Navas
    [PLEASE NOTE: Ads belong *only* in rec.photo.marketplace.digital, as per
    <http://bobatkins.photo.net/info/charter.htm> <http://rpdfaq.50megs.com/>]
    John Navas, Jul 19, 2003
    #4
  5. Phil Stripling

    Don Stauffer Guest

    Technically, a halftone (result of halftone process) was a sampled data
    system, and the dot was the basic picture element (pixel). They called
    them dots, but they were/are pixels. Now, this is ONLY true for
    halftones, not pictures printed on personal computer printers.

    Matti Vuori wrote:
    >
    >
    >
    > No, a dot is not a pixel, and never was one and never was called one.
    >
    >
    > --
    > Matti Vuori, <http://sivut.koti.soon.fi/mvuori/index-e.htm>


    --
    Don Stauffer in Minnesota

    webpage- http://www.usfamily.net/web/stauffer
    Don Stauffer, Jul 20, 2003
    #5
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