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.
That’s a very good trick about photoshop. Thanks for sharing
Here’s a couple more thoughts Julieanne…
For scanning flat work. I have rarely seen an advantage in scanning above 300 SPI. Prints just don’t contain microscopic information. Anything scanned at that resolution can easily be printed double its original size and maintain image quality.
For scanning film, I go back to my SprintScan 35 days which had a resolution of 2700 SPI. At that point you could see the grain in the film, so what would a higher rez scan do for you? I know scanners are going much higher, but I question the need.
I believe in scanning once, making a master file, and then downsizing to what is needed. With the time it takes to load the film into a scanner, you might as well grab all the information you can and avoid the possibility of having to handle the film more than once.
It would be really nice for Adobe to come up with a TWAIN replacement interface for us who still use scanners on a daily basis. ….hint…hint. 🙂
Later!
Thank you for your input Wayne – it’s always helpful to hear what our customers are doing in the real world! : )
Huh. That’s some good information. And I like that advice, Wayne. Thanks!