A digital "raster" image acquired from a scanner, a digital camera, or created directly in a "paint" application like Adobe Photoshop is made up of a mosaic of "pixels" (picture elements)."
Here is an original image at actual size:
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Here is a close up view showing the actual pixels that form the image:
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The physical size of the image is described by two numbers which can be expressed two ways:
1) The number of pixels per inch/centimeter.
and
2) The number of pixels in both horizontal and vertical dimensions.
Or:
1) The number of pixels per inch/centimeter.
and
2) The horizontal and vertical dimensions expressed in inches/centimeters.
Those are just two ways of saying the same thing.
Here is the original image with a dialog box showing its dimensions:
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Note that the dimensions have a "lock" icon beside them. This is because the relationship of pixels per inch (ppi) and vertical/horizontal size are "locked" together. Changing one changes the other as you can see in the below dialog boxes (click on image to enlarge):
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Note that as the resolution is changed (from 600 to 300 and 300 to 150 pixels per inch) only the density of the pixels changes, not the number of total pixels in the image, in this case 1412 pixels x 2028 pixels, therefore the file size remains the same. Put another way, each time the resolution in ppi is increased, or lowered, the physical image size changes but the total number of pixels forming the image (and hence the detail) remains the same.
Note that I use the term "pixels per inch" - ppi. Very often the term that is used is "dots per inch" or dpi. Technically the terms are not interchangeable - however, in daily usage, when speaking about digital images the terms are considered as meaning the same thing. You may sometimes hear the term "spi" - samples per inch. This refers to a scanner's resolution - i.e. it ability to acquire an image at so many samples per inch (e.g. 300 spi). Again, in practical usage, when speaking about digital images - ppi, dpi, and spi can be understood as meaning the same thing.
Interestingly, digital cameras typically do not have a resolution assigned to them.
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Instead a digital camera captures data based on the "megapixel" ability of its CCD sensor. For example, a 14.2 megapixel camera might capture an image that's 4592 pixels by 3056 pixels, which equals 14,033,152 total pixels. When you open the file into an image-editing program a resolution must be assigned to the file. Most programs, including Photoshop, use 72 ppi as the default resolution.
Background - halftone dots make the image reproduction
Because printing presses can only lay down 100% ink or 0% ink, digital images acquired from scanners, digital cameras, or created directly in "paint" applications need to be converted into a binary (on/off) format. This is done through a process called halftone screening. The result is that the image will be converted to dots of either 100% or 0% ink with the original tones being simulated, in this case, by the size of the dots. Bigger dots represent darker tones - smaller dots represent lighter tones:
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The fineness of the screen, and hence the level of detail in the original that can be preserved, is determined by how densely packed the dots are and is indirectly described by how many rows - or lines of dots are used per inch (or centimeter) to create the image. These virtual lines are highlighted in red below:
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The key thing to remember is that although the halftone image is made up of dots - the level of detail that it can reproduce is described in terms of lpi NOT dpi.
So, original image pixel density/detail = ppi, spi, or dpi. Halftone reproduction dot density/detail = lpi.
Of course, in order to pack more lines of dots into an inch - the smaller the dots become and hence the greater amount of image detail that is preserved.
40 lpi halftone:
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100 lpi halftone:
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200 lpi halftone:
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It is the relationship of how densely packed the original pixels are (see part 1) compared to the frequency of lines per inch of the halftone screen dots that determines what image resolution is appropriate for its reproduction in print.
The relationship between dpi/ppi and lpi for
grayscale images
The guiding principle for understanding what original image resolution (ppi/dpi) is needed compared to the halftone screen (lpi) that will be used is that the image pixels should always be more densely packed (ppi/dpi) than the detail resolving ability (lpi) of the halftone screen that is used.
To illustrate this principle I'll take a section of the same image at different resolutions (ppi/dpi) and reproduce it using the same 150 lpi halftone screen:
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Original 75 ppi/dpi - halftone screen 150 lpi:
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Original 100 ppi/dpi - halftone screen 150 lpi:
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Original 150 ppi/dpi - halftone screen 150 lpi:
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Original 225 ppi/dpi - halftone screen 150 lpi:
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This minimum required original resolution can be represented by the formula: 1.5 X lpi = ppi @ 100% reproduction.
Original 300 ppi/dpi - halftone screen 150 lpi:
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This ideal required original resolution can be represented by the formula: 2 X lpi = ppi @ 100% reproduction.
Original 600 ppi/dpi - halftone screen 150 lpi:
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The relationship between dpi/ppi and lpi for CMYK
images
As with grayscale images, the guiding principle for understanding what original image resolution (ppi/dpi) is needed compared to the halftone screen (lpi) that will be used is that the halftone screen should not reproduce the image pixels themselves but instead the tones the pixels represent. It is worth comparing these images to their grayscale equivalents in part 3.
To illustrate this principle, I'll take a section of an image rendered at different resolutions (ppi/dpi) that has been converted from grayscale to CMYK and reproduce it using the same 150 lpi halftone screen:
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Original 75 ppi/dpi - halftone screen 150 lpi:
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Original 100 ppi/dpi - halftone screen 150 lpi:
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Original 150 ppi/dpi - halftone screen 150 lpi:
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This minimum required original resolution for a CMYK image can be represented by the formula: lpi = ppi @ 100% reproduction.
Original 225 ppi/dpi - halftone screen 150 lpi:
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This ideal original resolution can be represented by the formula: 1.5 X lpi = ppi @ 100% reproduction.
Original 300 ppi/dpi - halftone screen 150 lpi:
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This maximum required original resolution can be represented by the formula: 2 X lpi = ppi @ 100% reproduction.
Original 600 ppi/dpi - halftone screen 150 lpi:
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The below table provides image resolution requirements for a variety of typical print applications:
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Image resolution "gotchas" – where things can go wrong
Whether you are targeting your images for AM or FM screening, there are at least three places where the resolution of the images may be accidently altered:
1) If the image is resized/scaled in the page layout application – it may no longer have an appropriate resolution:
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2) If the image is resized/scaled when the file is converted to the PDF format – it may no longer have an appropriate resolution:
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3) If the printshop's workflow is setup to resample incoming documents – they may no longer have an appropriate resolution. Most prepress RIPs are set, by default, to downsample incoming files to 300 ppi/dpi.
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thank you so much
ReplyDeleteWow very imformative! Much appreciated! I will use this on printfanatics.net forum
ReplyDeleteDear Gordon
ReplyDeleteI CMYK'ed and resized some large 72 dpi JPGs in photoshop to 300 dpi - WITHOUT resampling of course- so that its dimensions became close to 34 cm.
But I needed (for some pix) 9cm wide pics, so when I altered this dimesion - again withOUT resampling - the dpi automatically went up to something ridiculious like 1157 dpi.
How will this manifest in offset printing? Is there a danger of Moire patterns emerging? (There aren't many gradients in the pics but areas of flat color mostly).
Not sure what to do - should I downsample them in photoshop which will arbitarily decide which pixels to delete, or should I send these ultra-hi-res pix to the printers, and leave it to their RIP structure to downsample the images to match their lpi?
Would appreciate your views, thanks from India, where some printers still use coreldraw lol :)