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Adobe Creative Suite 5 Printing Guide 57

expressed in pixels per inch (ppi). The higher the resolution of an image

that’s a given size, the more detail that can be displayed.

36 ppi: 128 KB 72 ppi: 236 KB 300 ppi: 2.2 MB

Upsampling: Resolution increased

to 300 ppi from 72 ppi.

Image Resolution

e higher the number of the pixels per inch (ppi), the more

detail an image can display. More pixels per inch also means

larger image size. Increasing the resolution, or upsampling,

from 72 ppi to 300 ppi in Photoshop does not recreate

missing information. Note the soening of detail in the image

that has been upsampled in Photoshop, compared to an

image scanned to the correct size at the 300-ppi resolution.

The appropriate resolution for an image to be printed, based on the

final line screen, is 1.5 to 2 times the line screen, at or above its final

size. For example, an image to be printed at 150 lines per inch (lpi)

should be scanned to final size with a resolution of 225 ppi to 300 ppi.

There is no advantage to exceeding 300 ppi for an image to be printed

at 150 lpi; the excess information incurs unnecessary processing with

no visual improvement. Rather than do the math, most users elect to

use 300 ppi as a general target resolution.

It’s important to emphasize that, whatever the target resolution, the

image must be scanned (or photographed digitally) at or above its final

printing size. That is, an image intended to print at a size of 5 x 7 inches

and a line screen of 150 lpi should be scanned to 5 x 7 inches at 300

ppi. Scanning an image at 300 ppi, but at a size of 2 x 3 inches, with the

intention of scaling up the image to the correct final size of 5 x 7 inches

(either in Photoshop or in a page layout program), will compromise

detail after resizing. Photoshop can interpolate information, but this

results in the program’s “best guess” approximation of the missing

pixels. There is no substitute for a correctly sized original image of the

proper resolution.

Compared to scaling up, scaling down an image is less likely to produce

obvious loss of detail, whether the scaling is done in Photoshop,

Illustrator, or InDesign. Even though image data will be discarded, the

smaller size camouflages the loss. Plus, if you use the Bicubic Sharper

method in Photoshop when scaling down the size of an image (which is

recommended; this is discussed in the following topic), slight edge

sharpening is applied to compensate visually for the discarded data.

Actual versus Effective resolution

The best approach to image creation (whether scanning or shooting a

digital photograph) is to have the image at the proper size and

resolution before using it in other applications (or printing directly from

Photoshop). Generally, if you start with an image that is 300 ppi at a

given size, you have some leeway; scaling up to 125% in a page layout

probably won’t degrade detail in an obvious way. An image without

much detail (such as a background shot of a soft, gauzy sky) could still

be acceptable if scaled up to 150% in a page layout. An image with a

resolution of 300 ppi that is scaled up to 200% in an InDesign or

Illustrator document has an effective resolution of 150 ppi. In other

words, scaling an image up in InDesign decreases the resolution, and

scaling an image down in InDesign increases the resolution. Conversely,

a 300-ppi image that is scaled down to 50% has an effective resolution

of 600 ppi. Scaling down images in InDesign or Illustrator does not

reduce the amount of information included in an exported PDF, or an

outgoing print stream, as no pixels are actually discarded. Processing

time may increase slightly in the RIP. If just a few images are scaled

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Adobe 5.5 Design Standard Manual de usuario Pagina 59

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Adobe 5.5 Design Standard Manual de usuario Pagina 59

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