Does a 720 dpi printer actually print 720 true dots per inch?

Discussion in 'Digital Cameras' started by Paul D. Sullivan, Oct 1, 2004.

  1. I have a 5 megapixel image that has 426 dpi of actual data on a
    6"x4" print (2556 x 1704 actual pixels).

    If I have an Epson printer that does 2880 x 720 dpi, will that
    printer actually be able to print / resolve 426 actual dpi when
    it prints out the original image onto high-gloss photo paper?

    Or is it not able to actually print 426 dpi?

    I ask because someone told me that they thought the Epson could
    only resolve 360 dpi max, which is a divisor of the Epson print
    resolutions and that it could not resolve anything above that in
    actual printing.

    Any help would be appreciated.

    Paul D. Sullivan, Oct 1, 2004
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  2. Paul D. Sullivan

    gsum Guest

    You need to divide the number of dpi by the number of colours plus
    black to get the raw ppi measure as each printer pixel is a composite of
    the colours. Calculating the actual ppi measure is much more difficult
    as dither and anti-alias algorithms are applied to each pixel.

    Printers such as the Epson 1290 have a 2880 dpi setting but there is no
    visisble difference between 2880 dpi and 1440dpi and 2880 does not
    seem to be recommended by Epson. The maximum resolution of
    the Epson 1290 is about 150 to 180 ppi.

    gsum, Oct 1, 2004
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  3. Paul D. Sullivan

    bob Guest

    Not really.

    If it's a 4 color printer (CMYK), then 720/4 = 180 per inch, but at that
    "resolution" you can't resolve any colors. It take a good number of dots to
    create a single pixel. Most modern drivers use a stochastic dithering
    method, which makes calculating ppi somewhat less than straightforward.

    bob, Oct 1, 2004
  4. The printer driver will internally interpolate to 720 ppi, and then
    dither that to the printer's positioning accuracy. It will use
    smaller/variable droplet size and error diffusion dithering to mix
    intermediate colors from the available inks.
    No, the printer uses 720 ppi internally, regardless of what you offer,
    and will interpolate to reach that. That information can be obtained
    by software that interrogates the driver, which will then return that
    info. A program like Qimage uses that to optimize the interpolation
    and sharpening, and only then sends data to the driver which dithers
    without change.

    Bart van der Wolf, Oct 1, 2004
  5. The Epson printers certainly print the number of dots per inch they
    claim. I can see them with adequate magnification. (In an area of
    heavy ink loading they sometimes merge.)

    Each dot is *not*, however, a pixel; it cannot, by itself, contain the
    full range of colors and brightnesses. One of the reasons inkjet
    prints look so good is that they don't consist of simple pixels.
    David Dyer-Bennet, Oct 2, 2004
  6. Paul D. Sullivan

    andrew29 Guest

    It is, but you may not be able to see it.
    There's a box in the printer driver called "finest detail" that
    switches from 360 to 720 ppi.

    andrew29, Nov 12, 2004
  7. Paul D. Sullivan

    John Doe Guest

    Yes the printer prints 720 dots per inch. However, you need to remember that
    it takes more than one dot to make most colors. For example if you have a 7
    color printer than you need to divide the 720 dpi by 7 to get the printers
    effective resolution. This is also how I decide what DPI to have my images
    to print at. My Epson R200 is 5700 dpi and has 6 colors so when I print
    using it I try to have my images at 950 dpi. For a 720 I would aim for 120
    dpi. Now this isn't always possible and one must use what one has access to.
    For example when printing a 13x19" print on my Canon i9100 I am usually
    lucky of I have 200 dpi to work with.

    John Doe, Nov 16, 2004
  8. Paul D. Sullivan

    Owamanga Guest

    It's not that simple. Epson (and I believe others) use variable
    droplet technology, with (currently) 6 different droplet sizes from
    each of the 6 different tanks onto the same piece of paper. However,
    the largest 2 of those will definitely overlap the next pixel

    It's roughly (6 + no droplet = 7) x 6 = 42 different colors for each
    of the 720 pixels in a linear inch. (Really less, because the color
    gamut of some of those inks overlap, as does the larger pixel overlap
    with the next one)

    Bill Hilton came up with the following link that has some samples for
    you to print showing a 720 ppi print looks better than a 300 ppi, with
    files to download and see for yourself:

    I can see what you are getting at, but this logic is bogus. If I had a
    perfect printer with say 4 million color tanks, then by your
    calculation 720/4,000,000 = 0.000018 dpi which is silly. My 'perfect'
    printer would be much closer to 720ppi than your one that can only mix
    7 tanks by 6 droplet sizes each.

    It comes down to this. Printers need to dither, and their current
    effective full-color resolution is around the 240-300ppi mark.
    However, you should send the jobs at 720ppi (for Epson, 600ppi for
    Canon) because this is the driver's native resolution and assuming you
    have un-sharped the image, there will be high-contrast areas that the
    printer may be able to achieve towards 720ppi for.
    The nozzle check pattern is done on a single pass without moving the
    paper. If the paper can be moved by 1/2 the distance between nozzles,
    then starting from Epson's 180 dpi you can achieve 360 DPI. Do it by
    1/4 and you get 720DPI. (Watch an Epson head build the solid colors in
    no less than 4 passes).

    Realistically, that's all there is. The Epson's 1440 dpi does not
    relate to the paper feed direction, it's now offset timing the dots in
    the direction of head movement to give you 1440dpi along that line.
    There is a hell of a lot of overlapping dots at this rez.

    A single pass with all black heads firing will result in a gray line.
    To go Black, several more 'interlacing' passes are required.

    Use 1440DPI where the medium (and in my experience the internals of
    the printer) requires saturating with ink.

    You may ask, how can you ever get 1440dpi when you only send it 720ppi
    information? - or rather why try? Well, if they could make the dots
    smaller, a 2880 dpi printer would be better and 5760dpi or 11520dpi
    printer almost perfect for true 720ppi full color printing, giving
    enough space for dithering hundreds of different color dots to form
    each pixel.
    Owamanga, Nov 16, 2004
  9. Paul D. Sullivan

    Big Bill Guest

    Aren't you guys mistaking DPI for PPI when you sed the pics to the
    There is no way to correlate DPI and PPI. It just can't be done.
    Trying to do so is a fool's errand.
    Why? Becasue there's no way you can determine how many dots will be
    used to print a pixel. The driver will chose anywhere from 0 (zero) to
    the maximum the printer can print per inch for a given (user chosen)
    When you choose the PPI to send to the pruinter, what you're doing is
    attempting to get a reasonable number of pixels per inch on the
    printed image; 200 to 300 is usually considered adequate. The printer
    driver will translate this into dots, depending on the color and
    luminance of the pixels involved. The fewer pixels per inch the user
    stipulates, the more dots the printer will use to print each pixel,
    because that pixel will cover a larger area of the paper than would be
    the case with a higher PPI.
    Big Bill, Nov 17, 2004
  10. Paul D. Sullivan

    andrew29 Guest

    I'm surprised how often this comes up. There's an image at that shows just how good
    high-resolution printers really are. The image on the left is printed
    on an Epson 7600, and the upper grid lines are at the highest 720 ppi
    resolution -- i.e. 360 line *pairs* per inch.

    andrew29, Nov 23, 2004
  11. Paul D. Sullivan

    bob Guest

    wrote in
    In B&W, but only vertical.

    Try to make light grey or tan lines on a 5 degree angle and you probably
    drop to 1/3 to 1/2 of that...

    bob, Nov 23, 2004
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