by Julieanne Kost
The easiest way to calculate how large of an image you need to scan is to use the built-in function of your scanner (if it has one). Every scanner is different, but most of them give you the ability to select an area (some will even let you define the area or constrain the selection to a defined aspect ratio) and will calculate the resulting file size for you. Some will even show you the resulting size at a user defined resolution.
But if you’re not so lucky, then the best thing for you to do is to “cheat” by using Photoshop’s “New” dialog box as a calculator. Before scanning, decide at what size you will be printing the image. Then select File > New and enter the desired width, height, resolution, color mode, and bit depth that you will be working with. This will calculate the size for you and display it on the right side of the dialog. Then set the parameters of your scanner to scan that size (or close to it). This is easiest if you are going to use the entire scanned image without cropping. However, most of the time, you are either going to crop the image because you only want to use a part of it, or you are going to need to crop it because the original doesn’t match your intended aspect ratio. Regardless, when you are using your scanning software, there should be an option that allows you to select an area (hopefully it even lets you select an area based on a constrained aspect ratio) and will calculate the resulting file size.
Please don’t forget to take into account the bit depth and color mode in which you are scanning. Those gray scale files will be only a third the size of their RGB counterparts, and those 8 bit images are only 1/2 the size of their 16 bit equivalents.