Resolution Requirements
Mr. Toad's Requirements
Paper products: All raster (Photoshop) images at an effective resolution of 225-300 ppi, except for "bitmap" (one-bit) images at 1200 ppi
Disc Printing: All raster images at an effective resolution of 150-200 ppi, except for "bitmap" (one-bit) images at 1200 ppi
What is "effective resolution?"
If a 1" x 1" image is 300 ppi, and you blow it up to 5x5, then it's not 300 ppi anymore - now it's 60 ppi. Please don't enlarge a miniscule placed image in your page layout software to the point where the effective resolution is too low, and please don't download a 72 ppi JPEG from the web, upsample it to 300, and resave it for use in a layout. That's cheating, and you'll live to regret it - you can't make something from nothing!
What is resolution?
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| The noun "resolution" is derived from the verb "resolve: to render parts of an image visible and distinct" (American Heritage Dictionary, Third Edition, Houghton-Mifflin). The higher the amount of detail that can be rendered, the "higher" the resolution. The resolution of raster images is typically measured in ppi, or "pixels per inch."
All Photoshop images are arranged in a grid, or "raster". Each square in the grid is assigned a color value. Raster images which have a fine grid, with a high density of pixels per square inch of area, are known as high resolution images. There is no theoretical limitation to the coarseness or fineness of the grid; an image could have 1 pixel per inch, or 10, or 2 million. In practice, there are formulas for determining what resolution an image ought to be depending on how it is going to be used.
Have you ever seen a picture in the newspaper that looked "pixelated," not unlike the "low res" picture above? That picture had insufficient resolution for newsprint - it was obvious that there were details which should have been visible, but the image failed to resolve them. It was probably created at 72 ppi, which is appropriate for images that are to be displayed on a computer screen, but not good enough for print work.
The Formula for Calculating appropriate ppi for raster images
Continuous Tone (CMYK, greyscale, duotone)
The formula for calculating the appropriate resolution of "continuous tone" photoshop files is: 1.5x to 2x the "line screen". The paper products for the jewel case packaging are printed with a line screen of 150 lpi (lines per inch). Therefore, the appropriate range of resolution for continuous tone raster images is:
(1.5 to 2) x 150 lpi = 225 to 300 ppi
Going higher than the recommended resolution does not increase the print quality; all it does is cause an exponential increase in file size, and therefore, print time. 350 is a reasonable limit for overshoot. Higher than that and you're likely to incur additional costs for the extra time required to process your job.
For the disc, the formula works out like this:
(1.5 to 2) x 85 lpi = 127.5 to 170 ppi
It's OK to round this up to 150 to 200 - we're still not talking about very large files. 300 ppi would be complete overkill for disc art, though.
Bitmap (a.k.a. 1-bit)
Bitmap files, or more precisely, "1-bit files," behave differently from continuous tone images. They don't get sampled to form a line screen. They print at their actual resolution (sort of). 1200 ppi is a good number to shoot for. Go below 800 and you'll start to see some grungy edges. At 300, the grunge will be easily detectable, so it's important not to treat 1-bit images the same way that you treat greyscale or CMYK images.
In a 1-bit file, every pixel is either black or white - there are no intermediate shades of grey. The same is true for the "imagesetters" that are used to output film; either a printer dot will be filled with emulsion, or the area will be left clear. If a 1-bit file were exactly the same resolution as the imagesetter it was printing to, as in a 2540 ppi file printing on a 2540 dpi imagesetter, no further processing would be necessary, in theory. Each printer dot would correspond precisely to a sample in the 1-bit file.
In reality, it isn't necessary to goose up the ppi in your 1-bit files in order to match the resolution on an imagesetter, since imagesetters throw away information above 1200 ppi anyway. It is important that the imagesetters themselves have resolutions higher than 1200, but that's because the higher resolution enables a greater variety in the size of halftone dots, not because of superior edge detail. Intellectually, it may seem a little awkward to overlay a 1200 spi grid over a 2540 dpi grid, but in practice, it works just fine.
Are you sure that going above 300 ppi for continuous tone images doesn't help?
Yes. |
