Guidelines for Images
The accepted resolution of an image used in a printed piece, such as a brochure or booklet, is 300 dpi*. at size printed in the piece. To determine the dpi., divide the total number of horizontal pixels in an image by the width in inches that the image will be printed at and then do the same with the vertical pixel count and the height of the image when printed. The smaller of the two results will yield the effective resolution. Now, please do not stress out if the resolution of your image isn't exactly 300 dpi. There is an acceptable range. You can get away with as little as 225 dpi. without the image being perceptibly soft and up to 400 dpi. if you want to push the other end of the envelope. There is no perceivable benefit to saving an image over 400 dpi. at the size printed when offset printed: you are only creating large cumbersome files without any visual improvement to the print quality. The image below illustrates how resolution effects image quality. Make sure to check out the sections on color space and Adobe Creative suite settings as well.
*dpi = dots (pixels) per inch
Large-format Printing (Displays, Banners and Signs):
Resolution requirements for large-format output vary depending on the final output size of the image. Smaller images require a higher output resolution, because someone would need to come up close to see the image in detail. Large scale images can have lower output resolutions, because the viewer would need to stand several feet/yards back to take in the image. Images on billboards seen from an interstate highway may only be 10 to 20 dpi., but look fine viewed at that distance. Example: 300 dpi. for an 8x10 inch image - 72 dpi for an 8x10 foot image. Higher resolution is always better, but not always necessary and can create some very large and cumbersome files. The photo below illustrates the point. Up close, its easy to see the difference between the three different resolutions. However, if you back away from the screen a few feet, you can not see the difference between the 300 and 150 dpi. flags and if you back away about 10 feet or so, you can not see the difference between the 72 and 300 dpi. flags. Make sure to check out the sections on color space and Adobe Creative suite settings as well.
Guidelines for Images
Color reproduction is the trickiest part of any print project. Not only are there hundreds of possible paths taken from capturing the color of the original to the color produced in the final printed piece, there is also the subjective element of the human eye and the way each of us perceives color. There's not much you can do about the human element: varying degrees of color blindness, or two people arguing over whether a certain shade is orange versus red, or blue versus purple. However, it is possible to at least keep that color you insist is orange consistant throughout the various steps of your particular production process.
If you want to achieve consistant color between web, displays (large-format inkjet), or offset printing your images need to be profiled to the smallest color space.
When producing a piece to be offset printed, such as a booklet, brochure, or business cards, your images and vector art should always be setup as CMYK using Web Coated SWOP ver. 2 profile with "perceptual" rendering intent. You should convert everything to CMYK before editing the files. As shown in the figures below, the RGB color space offers a much broader spectrum of color than CMYK. When the image is converted to CMYK, the RGB colors are scaled back to fit within the limits of the CMYK spectrum and there are a multitude of paths that can be taken to accomplish the conversion. Each printing company and the various programs they use will all follow different methods to accomplish the conversion, so if you do not convert the files yourself beforehand, the final color can vary widely from job to job.
If you are starting with RGB images, the best practice is to first convert the images to Adobe 1998 RGB before converting to CMYK using the Web Coated SWOP ver. 2 profile. You will get the least amount of color shift this way. Also, converting directly from another RGB color space to the CMYK - Web Coated SWOP ver. 2 space will yield different color values. Make sure to check out the sections on resolution and Adobe Creative suite settings as well.
Large-format Printing (Display, Banners, Signs, Fine-art Reproductions):
Our large-format inkjet printers are capable of reproducing a much larger color gamut than an offset printing press, therefore, its best to provide the files to us in RGB, using the Adobe 1998 RGB profile. CMYK , using the Web Coated SWOP ver 2 profile, is fine if your files are already in that format. There is no point converting CMYK back to RGB, other than making the files a bit smaller. Since the CMYK space is smaller than the RGB and fits inside, the color will stay the same and nothing will be gained (see the figures below). Make sure to check out the sections on resolution and Adobe Creative suite settings as well.