Understanding Photoshop is a biweekly column that takes an in-depth look at how digital photographs are built and manipulated. It is a college-level course in plain English for free at Photofocus. To learn more see this article.

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People often get confused when determining which settings to scan with. Too little information and the picture goes soft. Too much information and the scanner slows to a crawl. The answer is to know your intended output resolution as well as your device.

Color printer isolated on white

photo by Avantgarde

Determining resolution

In this example, let’s say you need to create a 20-inch-wide poster. You also intend that it will be printed on a high-quality press requiring 300 ppi, use this calculation:

20 (inches) 300 (ppi) 1.25 (pad for flexibility) = 7500 pixels

Do not adjust your scanners dpi (or ppi) settings. Rather, crop the image after running a preview scan. You can then adjust the scanners resolution by looking at the output size of the scanned file. As you adjust the output file size, the scanning software will automatically determine the appropriate settings for samples per inch. All scanners tell you just how many samples you are about to capture. Looking at these numbers gives you a truer sense of the end result. Total pixel count is much more important than dpi, especially when scanning images of various original sizes.

Scanning previously printed items?

If you are scanning an image that has been previously printed in a book or magazine, you may need to de-screen it. Look to see if your scanner offers a hardware-based de-screening option.

image by magraphics.eu

Scanner operation

It is safe to say that every scanner model is a little different. Hardware manufacturers must write software that allows the scanner to interface with your computer. When choosing a scanner, be sure it works with your computers operating system (always check the box or manufacturers Web site carefully). Here are the steps to operate a scanner:

  1. Before scanning an image, install the software and drivers needed by your scanner. These are usually included on a disc provided by the manufacturer or are offered for download from its Web site. This software runs as an independent program, but Photoshop can open the resulting scans. Some scanners can also work directly from inside Photoshop by choosing File > Import > Images from Device, so be sure to see your scanners documentation.
  2. Ensure that the scanner is lying flat, or you may get misregistered scans.
  3. Place your photos on the scanner and make sure they are straight. Use the edges to help you maintain parallel edges on your photos.
  4. Run a preview scan first to check image placement and details.
  5. If your scanner allows it, set the white and black points before scanning. This is accomplished by making a preview scan, and then using your scanners software to identify a black and a white point in the image. You can then use Photoshops color correction tools to adjust the white and black points as well as make additional color changes. Every scanning software program is different, so be sure to read the documentation included with the scanner or on the manufacturers Web site.
  6. Scan slightly higher than the quality you need; for example, scan at 300 spi for newsprint, even though you may only deliver it at 170 ppi. The extra pixel information allows you to zoom in for further corrections. It also gives you extra pixels in case you need to crop the image.
  7. Save to formats such as TIFF (Tagged Image File Format, a standard in the print industry). This file format is efficient for storage and supports lossless compression to reduce file size. The Photoshop (PSD) format is great for layered files but is not as efficient for single-layer files. Always save the file using the appropriate extension for your file type.

Capture more than you need

Theres no need to overdo it, but I always recommend capturing two to three times more data than you will need. For example, if you will be outputting a Web graphic at 800 600, you should capture at least 1600 1200 pixels to start. Having the extra pixel data will give you more details to work with when zooming in for touchup. It also allows you to make decisions about cropping and reformatting.

Crooked scan? Fix it later with PhotoshopFig 03_13 Crop

In order to save time, I like to load as many images onto the scanner bed as possible Then, if you get crooked photos when scanning, you can use a Photoshop automation command to automatically crop and straighten your images.

Simply open the file and then select File > Automate > Crop and Straighten Photos. You’ll find two Crop and Straighten demo files here and here.