This photo was scanned at two different resolutions. The image on the left was scanned at 300 SPI and the image on the right was scanned at 72 SPI. Examine the detailed enlargements to see the impact of different scanner settings.
In Chapter 3, we looked closely at the process of acquiring digital images. If you skipped ahead or just skimmed that chapter, go backa solid understanding of those concepts is required. Quite simply, you must be aware of the ability of your hardware to ingest information. Be sure to know what your scanner or digital camera is capable of.
In previous chapters, we also briefly discussed output requirements for different formats. The second part of the image-sizing puzzle is a clear understanding of output requirements. What resolution does your printer need? Are you sending the image to a service provider such as a developer or commercial printer? There are a lot of choices you'll need to make, but they are based on where the image needs to end up. Do not make assumptions when starting a project. Know your destination, and you will know which path to take.
If you are acquiring a digital image, then be sure to capture enough pixels. If you are trying to make a 5 x 7 inch print and need 300 DPI, then do the math before shooting. Take the inch size and multiply it by the print resolution. In this example: 5 x 300 = 1500, and 7 x 300 = 2100. Therefore, 1500 x 2100 = 3,150,000, which is about 3.1 megapixels. To allow for cropping, you may want to shoot at an even higher resolution.
If you know the LPI requirements of your output device, then it is relatively easy to determine the resolution you'll need. In general, you'll want a resolution that's 1.5 to 2 times the screen frequency you'll use to print. For example, if you need a 3 x 5 inch image at 150 LPI, then you'll want to use the following settings to scan.
150 LPI x 2 = 300 DPI
300 DPI x 3 inches = 900 total pixels needed
300 DPI x 5 inches = 1500 pixels needed
Scan with a pixel dimension of 900 pixels x 1500 pixels
Remember: To allow room for cropping, you may want to scan at a higher pixel count. If you feel unclear on determining the right scanning resolution, be sure to revisit Chapter 3, "Acquiring Digital Images."