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).”
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.
2) The number of pixels in both horizontal and vertical dimensions.
1) The number of pixels per inch/centimeter.
2) The horizontal and vertical dimensions expressed in inches/centimeters.
Those are just two ways of saying the same thing.
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.
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:
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 linesof dots are used per inch (or centimeter) to create the image. These virtual lines are highlighted in red below:
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.
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.
This minimum required original resolution can be represented by the formula: 1.5 X lpi = ppi @ 100% reproduction.
This ideal required original resolution can be represented by the formula: 2 X lpi = ppi @ 100% reproduction.
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.
This minimum required original resolution for a CMYK image can be represented by the formula: lpi = ppi @ 100% reproduction.
This ideal original resolution can be represented by the formula: 1.5 X lpi = ppi @ 100% reproduction.
This maximum required original resolution can be represented by the formula: 2 X lpi = ppi @ 100% reproduction.
The below table provides image resolution requirements for a variety of typical print applications:
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:
2) If the image is resized/scaled when the file is converted to the PDF format – it may no longer have an appropriate resolution:
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.
For more details, check here.