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Adobe Photoshop! DPI unit , how ?

Posted on 1997-08-30
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Last Modified: 2008-03-06
Hi...
I was wondering how much 2400DPI are pixel/inche ?
i mean , is a dot a pixel ?

i mean , in my photoshop settings image menu , what is the exact value i put in the request : pixel/Inch for a 2400 DPI final release ?
 
thx !!
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Question by:SkizoWalker
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by:weed
ID: 1540485
well...i think your answer lies in "DPI" which stands for "dots per inch". If you want a 2400 dpi final image you put 2400 into the settings. note that the resolution of a monitor is 72 dpi. Make your calculations accordingly. thus...the larger your dpi number the larger the image appears on screen but the smaller it prints.
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by:SkizoWalker
ID: 1540486
i didn't ask my question correctly .. So how much 2400 dpi does in Pixel per inch ?

DPI = Dots per Inch
 
1 pixel = 1 dot ???

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by:weed
ID: 1540487
how much what? you mean much does 2400 dpi equal in pixels per inch?

youre trying to convert between two things that are unrelated. pixels are one thing. dots are another. Pixels are on a screen, dots are on the printed paper. a 2400 dpi image will equal 33.33 inches on a screen because a screen is oly 72 dpi. if you were to print your screen there would be 72 dots per inch on the paper.  c?
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by:kingrea
ID: 1540488
my question would be which film/imagesetter/plate/press combination is capable of 2400dpi? as technology administrator for a large printing company, this info would be most helpful in planning for the future. certainly this configuration would exceed even stochastic printing, the technology for which is still pretty flaky. am i to understand that image scans are made in excess of 6000ppi in order to support printing at this level? WOW! fill me in!
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gormenghast earned 0 total points
ID: 1540489
I am not sure exactly what you are asking....but

If you are considering outputting an image to an imagesetter at 2400dpi (more likely 2540dpi) then if the screen ruling required on film was 150lpi (lines per inch)--most common screen ruling for litho printing--then your image should ideally be 300ppi at full size. This is based on the ratio of 2:1 for image resolution to screen ruling. If you wanted 175lpi on final film you would require an image resolution of 350ppi.
This means that for every row of dots produced by the imagesetter there are two pixels in your image which means there will be no pixelization on final film.
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by:rickyr
ID: 1540490
Hi..
You gotta give it to gormenghast here. He's got it right.
Have a look at my profile and you'll see my experience in this field.
Oh! I think kingrea may be taking DPI (Scan Res) for LPI (Halftone).

regards
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Expert Comment

by:Farfrae
ID: 1540491
I believe you are confusing pixels with output resolution. First, your screen-image is composed of pixels (one each of red, green, blue) at 72pixels per inch. Your Output will be in dots on film. At 2540 DPI (1000Dots per cm) you should expect to have 256 shades of grey (UK spelling) per colour (Cyan, Magenta, Yellow, Black). Traditionally this has been done using a screen ruling, with each of the colours being output at different angles. Each 'dot' is actually a grid of 16 x16 laser spots. The intensity of each dot depending on how many spots are 'black'.(Remember that each separation is rinted black). To achieve the 256 shades of grey, a formula is used, which means that a screen of 150LPI (lines per inch) can be used.
In order to keep scanned file sizes to a minimum without pixellation, a good rule of thumb is that the Scan DPI should be twice the Output screen ruling. I know this is complicated, and I perhaps haven't explained it as well as I could.
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