Oversampling:
Oversampling is the ratio of pixels needed to form each halftone
printing dot. In order to get good detail and smooth tonal transitions in the
screened image the number of pixels of the digital file should exceed the screen
ruling by a factor of 1.5 to 2.0. This is known as the oversampling ratio.
Experts with a great deal of experience have suggested that an oversampling
ratio of 1.8 is ideal. How that translates into pixels per inch output
resolution depends on the type of printing
Most offset printers use 150 line per inch halftone dot patterns on
coated paper (i.e., magazines). In practical terms that means if an imagesetter
is producing color separated films with 150 dots per inch output for a magazine,
the digital files sent to the imagesetter should ideally have 270 ppi (i.e. 150
x 1.8 = 270) ppi.
Fine arts books can be printed with screen rulings of 200 or
greater. Newspapers use lower resolution. B&W newspapers are usually 85 dpi.
I'm not sure what the standard for color on newsprint is. As for posters, the
line ruling really depends on the viewing distance. Even billboard with dots
the size of tennis balls (a line ruling of 0.33 dpi) will look continuous tone
if viewed from far enough away.
Resampling:
Resampling is what you do in PhotoShop to increase the number of pixels
in the original image to enlarge it. This is the point where many eyeballs roll
back and the room starts to spin, but its really pretty simple using Photoshop:
- Crop the camera file as needed to the proportions needed for the final product (none or as little as possible is best)
- Open the image size window (Image - Image Size)
- Enter the desired output size (either height or width) and check proportions. Recrop if necessary to correct proportions (go back to A).
- Enter the resolution required for the output device (e.g., Imagesetter @ 150dpi w. 1.8 oversample ratio = 270)
- Select "resample image - bicubic" and press OK
Photoshop will then increase the number pixels in
the original file to the number required need for 1.8
oversampling during 270 ppi imagesetting. Of course you
won't have a clue what actually happened, but if the result
looks OK who cares?
Should you do the resampling or should the printer?
I said at the beginning as a printer I prefer to get digital
files as close to their original native state as possible.
Since cropping and sizing is part of the design process
resampling is the exception to this rule. Crop, resample,
and retouch, but leave the color correction and contrast
adjustment to the printer, or do it in close consultation.
Keep in mind that to size and resample a file you need to
know the screen ruling the imagesetter / printer will use
(e.g. 150 dots per inch).
Remember that the entire color reproduction
calibration process, if done correctly, starts with the
printed sheets and works backwards calibrating monitors and
proof printers. In general, a printer which handles both the
imagesetting and printing should produce better results than
using a prepress house to make the film, then selecting a
separate printer. Ask your printer if they can provide you
with the ICC profile for the stock and ink they plan to
print your job with. If they say, "What's an ICC profile"
look for another printer and tell them to read the next section.
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