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Scans
& Graphic File Formats
What type of original is best to scan? For most color work, a positive transparency, slide, or reflective piece of art, such as a water color on paper, is best. Color negatives are not good for scanning, a photographic print should first be made from a color negative. Because a previously printed image has already been screened during the printing process, avoid rescreening of printed images. (Not to mention possible copyright infringement issues.)
Images taken on digital cameras are acceptable as long as you follow the DPI guidelines set out below. Use a camera with at least 5 megapixels. Also use the higher resolution/less images setting of a digital camera for best quality.
What
DPI setting should I use for my scan? DPI
is your scanner's resolution defined in dots per inch. Printers use screens
to make printing plates with resolution defined in LPI (lines per inch). This
is also referred to as the line screen or halftone screen. The rule-of-thumb
method for figuring the best setting to achieve sufficient quality scans is
the ratio 2 DPI:1 LPI. Knowing what line screen you will be using for the
halftone, double that number, and you will have your scan setting. Here at
ProGraphics, we typically print at 175 LPI, or 175 line screen. Therefore,
a 175 line halftone would be scanned at 350 DPI at the final size. For most
scanners, using a higher DPI will only make a bigger file without any quality
benefit. However, black and white bitmap line art requires much higher resolutions,
such as 1200-2400 DPI.
If you are creating an image in Photoshop, or a similar image pixel editing/painting program, set the DPI as if it were a scan. In other words, if you intend to print at a 175 line screen, set the DPI at 350. It is wise to do this during file creation rather than starting at low-res and resampling later, since resampling upward degrades the image.
FPO
(For Position Only).
You
may be doing all of your own prepress work except for a high quality scan.
You can submit your file with a For-Position-Only (FPO) low resolution scan
to show size and cropping and we can do the scan, place it in your file and
output a proof. Also, if necessary, we can have a high-res scan put on removable
storage media for you to place in your file yourself if you have specific
retouching, cropping or other issues that you wish to handle on your system.
Graphic File Formats
Graphics file formats include: EPS, CT, DCS, TIFF, PICT, BMP, JPG, BMP, GIF and others. Also, graphics applications have their own native file formats: a Quark Xpress file, an Illustrator file, etc.
Bitmapped
graphics are graphic files made of dots/pixels and are created in a paint
or photo program such as Photoshop or scanned. Bitmapped graphics saved as
composite EPS (Encapsulated PostScript) files are most common and provide
excellent reliability. The DCS form of an EPS file works, but needs to be
merged prior to output. TIFF (Tagged Image File Format) files work, but with
the warning that if an area of the image is pure white, the transition to
white may not be as smooth as required, and jagged or pixelated edges may
occur. To help smooth the transition to white, select a background of white
(a background of none is usually the culprit) in the picture box of your page
layout program. TIFF files are at their best when filling a picture box with
an outline rule, or frame specified.
Vector graphics are graphic files made of mathematically defined outlines (versus individual dots/pixels) resulting in scalable smooth-edged images. Vector graphics are created in a drawing program, such as Illustrator or Freehand and they should be saved as EPS files. If you set type in these EPS files, you must provide the screen & printer fonts or convert the text to outlines. If you have a placed EPS inside of an EPS be sure to save the embedded EPS with the file or the link may not work.
Use caution when specifying line widths. A hairline rule will often print acceptably on a laser printer but may be so fine that it is barely visible when output at high resolution on the printing plate.
GLOSSARY
Bitmap - A graphic file made of dots/pixels, Photoshop and other paint or photo programs produce bitmap files
DPI - Resolution defined in Dots Per Inch
EPS - Encapsulated PostScript, may be vector or bitmap
LPI - Resolution defined in Lines Per Inch; halftone value, as in 175 line screen.
Pixel - Smallest picture element designated by res or DPI
CT - Continuous Tone format of a bitmapped image
TIFF - Tagged Image File Format
Vector - A graphic file made of mathematical outlines (versus dots) resulting in a scalable, smooth-edged image. Illustrator or Freehand are typical of vector drawing programs.
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