Photoshop is a "Raster" based imaging program, meaning that when you have an image, the image is actually made up of little dots. This is different than a "Vector" based imaging program, which constructs it's image through little lines or vectors. (An example of this would be Adobe's Illustrator or Macromedia's Freehand)
When we speak about image resolution, we are speaking about the amount of information that a file contains or the dots that make the image. Generally speaking, the more information (dots) there is, the high the image quality. Image resolution is measured in "Dots per Inch" (abbreviated DPI) or "Pixels per Inch" (abbreviated PPI).
If you are creating an image to be printed, it is necessary to have an image file that is at least 300 dpi, to retain the maximum image quality. For any image that you may wish to print, please scan at the biggest size (in inches) you may wish to print it at 300 dpi.
When an image is displayed on a screen or monitor, it is displayed at 72 dpi, so if you are scanning something to be displayed on the web, there is no need to capture that additional information, as it will not be displayed anyway.
When scanning an image, you must first consider the limitations of the machine. Our scanner is an Epson 3170, which has an optical resolution of 3200 dpi @ 100%. Any values that the scanner can produce above that is through "interpolation" or "upsampling", which is when the scanner/scanning software generates fictitious information based on the information it has already captured. Photoshop can do the same thing. We also have a program called Genuine Fractals, which uses fractal algorithms to add information to the scan. This is the best way to add information, rather than in the scanning software. To this end, you are better off scanning no higher than the optical resolution of the scanner. Interpolating is a way to get more information, but the information is generated and of lower quality than the original. Therefore, it's best to not interpolate unless absolutely needed to print larger. And, the more information you have to start with, the more effectively you will be able to interpolate.
In Photoshop, we determine the image size by going to the "Image" menu and choosing the "Image size" option. This dialogue box will give you two different ways of thinking about the image. The first is the absolute pixel dimensions and the second is "document size" which is the DPI count in conjunction with the image size. If you were to multiply the size in inches by the image resolution, that is the number that is the pixel dimensions.
When scanning, it's best to scan at the highest quality you might ever print the image, and then scale it down as needed for different output forms. The printer that we will be using later in the course is an Epson 1280, which can print 13 inches wide by however long your paper is. It's easy to "throw away" information, but once it's gone, it's gone ... and you can't get it back. When you make a scan at the highest quality, I find it best to create a "master" file. I designate my master files through file naming conventions, for example a raw scan of one of my UFO images might be "UFO_43_m.psd", with the m designating Master. If I were to make a smaller version of it to make a small print, I would scale down the master image and save that smaller version as something different. ONCE YOUR MASTER FILE IS 'PERFECT' NEVER LOWER THE RESOLUTION. You may wish to print it bigger later. Think of the Master File as your perfect digital negative, from which you'll want to print again in the future.
As you are scanning for the first project, you will be using the scanning software as shown in class to correct your images as you scan. Do not yet begin to correct your images in Photoshop.
Questions? Shoot me an e-mail.