RESOLUTION
Scanning it the process of converting a printed, photographic or 3D object into digital format so in can be editied and incorporated into digital media. the key to getting a good scan is understanding your scanner's capabilities and selecting the optimal scanning resolution.

DPI vs PPI
Clarifying the difference between these tems can help in understanding the scanning to output process. DPI or dots per inch, should only be used when refering to the output of a printer. Laser or inkjet printers spray or deposit dots of color to recreate digitial images. When we scan and edit and view images on our computer systems we are working in pixels so we refer to the resolution of these images in pixels per inch.

The quality of printer output you use will determine the optimum resolution setting to use when scanning an image. The first question to consider then is what is my printer's output resolution? Most inkjets and lasers printers have a range of 150 to 1440 dpi which is selectable. Just to confuse the issue we have another output scale that's used in commercial printing. Press output resolution is measured in lines per inch or LPI. LPI measures the spacing of dots used to create halftone images.

Here's the issue. There is an optimum scanning resolution for any output device. To have lower than optimum resolution will not take advantage of the printer capability and you will loose detail and color quality. The printer makes up the missing information in a process called up sampling. While it seems logical that more is better, the same problem occurs if resolution exceeds the printer's capability....data will be removed in a down sampling process. Here are some guidelines to keep you in control of images and printing output:

RULES OF THUMB
Always know the printer's output resolution in DPI or LPI (commercial printers will be glad to assist)

Choose high quality original images with good tonal range

Divide DPI output by 3 to get optimum final image resolution
(note: you may want to scan originals at a higher resolution for scaling or editing but resample in Photoshop for final output)
Inkjet or Laser Printers are often set for 600 DPI output 600 divided by 3=200 PPI

Multiply LPI output by 1.5 to get optimum resolution
Newspaper presses typically print at 85 LPI (85x1.5=127 PPI)
High end printing presses are generally in the 150 to 200 LPI range (200 x 1.5=300 PPI

Scanning at 150 to 200 PPI will generally achieve the best all around results in class.

Scanning The Rest Of The Story

Before you finally scan a photo or image, a few other decisions must be considered. What type of image are you scanning? What is the final image size you’ll need? What’s the quality/resolution of the image you’re scanning?

Photos (real continuous tone photographs not out of books or magazines)
Original photos are either in printed or transparency form. This is the best source of images which can be digitized without pesky moiré patters and can be scanned at high resolution quality. Transparencies (pos or neg) are the very best image source.

Printed Photographs (from magazines or books)
Photos gathered from books or magazines can be challenging to scan. Because they are printed on various quality papers they will be fuzzier due to the printing process. These images are almost always halftone images made up of a series of overprinted dots from the CMYK print process. This pattern of dots typically becomes more accentuated when the image is scanned. This imperfection is called a moiré pattern. It can be improved in Photoshop but typically not eliminated without significantly effecting the quality of the image. Adjusting the scan angle, making channel corrections with the despeckle and blur filters can help.

Line Art (black and white solid line drawings that have no gray tones)
Line art can be challenging to scan and retain detail. Two options that seem to work best are to scan at your output device dpi resolution or scan in color and convert or desaturate to a greyscale image in Photoshop.

Image Size It’s best to scale your image during the scanning process to the maximum size you’ll need. Fine tuning can be done in Photoshop, but you will capture the most detail when you create your original scan. Be sure to consider the resolution of the document your bringing the image into because it will resample to "fit" that resolution. So…2 x 2 inch 72 ppi image will be 1 x 1 inch if brought into a 150 ppi page because it only can display enough information to fill the smaller space. Images can be resampled in Photoshop so you could take your 2x2 image and resample to 150 ppi and retaining the original dimension….image quality will however suffer because an interpolation model is used to add pixels to the image.

Scanning Checklist

Open Photoshop on one of the stations that has a scanner
Place the image you want to scan face down on the glass scanner bed/close lid
File>Import
(A plug-in has been loaded for your specific scanner model that will appear hear)
Select Twain Acquire or Vista Scan
Find the Preview button and select (this will scan the entire scan area)
Select the desired scan area with the selection tool (marquee tool)
Choose scale (usual default is 100%)
Choose desired resolution (150 ppi is a good average for most printers)
Select Scan
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Save your file on your zip disk or on the server SCAN FOLDER
You can retrieve documents in the SCAN FOLDER at your workstation