Image Resolution

Vector graphics are resolution-independent—thus, their resolution is determined only by the output device. Because vector elements are mathematically-defined, scaling (enlarging or reducing their size) simply requires modification of their component mathematical descriptions.

Whereas vector graphics are resolution-independent, raster images are resolution-dependent—the number of pixels that occupy a given space must be defined. Consequently, raster image resolution is specified in pixels per inch (ppi). However, although the term is not effective, image resolution commonly is referred to in dots per inch (dpi)—dpi more appropriately is attributed to device resolution or output resolution, where the number of dots an output device is able to produce within an inch represents the resolution of the device.

By dividing the number of pixels in the height and in the width of a raster or bitmap by its resolution will determine the physical size of the image—e.g., a 300ppi raster image that is 900 pixels wide and 600 pixels high is 3 inches by 2 inches in size: 900 pixels ÷ 300ppi = 3 inches wide 600 pixels ÷ 300ppi = 2 inches high

Monochrome (1-bit) image resolution for images intended for lithographic print is 900-1200ppi. In general, monochrome image resolution should be equivalent to the resolution of the intended output device. Output resolution for computer-to-plate devices (i.e., platesetters) typically is about 2400dpi; however, because no significant visible improvement is realized when 2400ppi and 1200ppi monochrome images are compared, the range for monochrome image resolution is suggested as 900-1200ppi.

Tone image resolution is recommended as 300ppi. This recommended resolution, as well as ranges suggested for other file types, assumes that images have been scanned at the same size intended for print (e.g., the typical image width is approximately 20 picas, or 3 1/3 inches, for a single-column image that will appear in a double-column standard size publication).

Combination tone image resolution is recommended as 500-900ppi. Because of text and/or line art components, sufficient resolution is required for combination tones to counter the effect of aliasing (i.e., stair-stepped appearance or "jaggies"); however, higher resolution equates to larger physical file size. Consequently, for combination tones, a compromise is achieved that balances the issues of aliasing and physical file size.

During the process of halftoning, series of pixels are translated to halftone dots of varying sizes and position. Improved results are achieved when more than a single pixel is available to describe the density of a given halftone dot. Consequently, a quality factor can be assumed to enable determination of the suggested resolution for tone images.

For tone images that will be halftoned at 133lpi (lines per inch) or higher, a quality factor of 1.5x is applied. For images that contain geometric objects: lines, repeating patterns, diagonals, etc., a quality factor of 2x the halftone linescreen is beneficial. Below 133lpi, a quality factor of 2x is recommended, since halftone conversion to proper densities is less accurate for lower linescreen rulings.

Because halftone linescreen usually is not known at the time of scanning, 300ppi is assumed to be a reasonable guideline for grayscale or color tone image resolution. Images that have been created or modified for Internet display, typically 72ppi, do not possess sufficient resolution to enable effective print. Employing the quality factor and subsequently adhering to image resolution recommendations ensures an appropriate balance between print quality and physical file size.

original = 266dpi low res = 72 dpi NOTE: When clicking between the high resolution image and the low resolution image, notice the differences in the edges of the cells, and the differences in the text at the bottom of the image.

Often times images are captured from Web sites or other methods that store images at 72 dpi (monitor resolution). When this low resolution image is printed, image detail will be lost, and the image will appear jagged.

Raster images can be classified as either monochrome, tone, or combination tone. It is important to maintain minimum resolution settings for each file type. Low resolution images are one of the leading causes of art resubmission and schedule delays.

Monochrome (1-bit) images, typically scanned from line art and/or text originals, are comprised of a single bit of data. Since each bit (binary digit) can be represented only as either a zero or a one, pixels within a monochrome image can have only two states: black or white. Monochrome images generally require higher resolution (more pixels per inch) than tone images in order to prevent aliasing (stair-stepped appearance) of diagonal lines. The suggested minimum resolution for this type of image is between 900 and 1200 dpi. See the chart below for details.

Tones, typically captured from continuous-tone photographs, are comprised of 8-bit data (represented as 256 different levels for grayscale images). Color tones contain eight bits of data per channel (i.e., per color); thus, a CMYK tone contains 32 bits of information (and 256 levels each for cyan, magenta, yellow, and black). The suggested minimum resolution for this type of image is 300 dpi. See the chart below for details.

Combination tones contain both tone and text/line art elements. Consequently, like tones, combination tones are comprised of 8-bits of data per color channel. Thus, combination tones are saved as either grayscale or CMYK color mode. Because combination tones contain text/line art elements, however, the level of resolution employed must be compromised to address aliasing versus physical file size. The suggested minimum resolution for this type of image is between 500 and 900 dpi. See the chart below for details.

TYPE	EXAMPLE	FORMAT	COLOR MODE	RESOLUTION
B+W Line art		Tiff	Monochrome 1-bit	900 - 1200dpi
Halftone		Tiff	Grayscale or CMYK	300dpi
Combo (Image + Type)		Tiff/Eps	Grayscale or CMYK	500 - 900 dpi

We have only covered half of the resolution story on this page. To complicate matters even further, scaling of artwork must also be considered in the resolution formula. The result of scaling is known as effective resolution.

   See also  "Conquering Inadequate Image Resolution"