To get the best possible scan from your scanner, make sure you choose a scan resolution that evenly divides into the optical resolution of your scanner. For instance, lets say you have a 300dpi (optical) scanner. You would want to choose a scan resolution of 300, or 150, or 75, etc... You would not want to choose 265, or 115, or 80, etc... You also must make sure that the scaling factor is set to 100%, so that the scanner software is not changing the actual resolution used for the scan.
Now you are probably asking yourself, why does it matter?
All scanners have a CCD array that convert light and color into digital pixels in an electronic image. The CCD array has a fixed (or optical) resolution. So, when you ask the scanner to set an odd scan frequency it has to resort to using a mathematical formula to interpolate the pixel values using bilinear interpolation. It is kind of like trying to evenly divide 3 grapes among four people! Using an even scan frequency, the scanner simply reads the value for each CCD element, or every other element, or every 3rd element, and so on. The results from an even scan frequency will not have any visual artifacts resulting from interpolation. An added benefit is that your scans will be much faster since the scanner does not have to mess with all that math while scanning.
Yeah, but my image is not the right size if I do that!
You are right. You want to choose an even scan frequency that will give you a larger sized image than you need, if possible. You will also need image editing software that allows you to resize an image using bicubic interpolation. Programs such as Adobe PhotoShop can resize an image using bicubic rather than bilinear interpolation. The advantage is that you don't get any artifacts from bicubic interpolation, only a softer image. Any sharpening routine (like UnSharp Mask) will bring back the image clarity. Back to the idea of a larger image than you need... When you scale the larger image down to the size you need you are throwing away pixels that are not needed. This is much better than scaling an image up electronically. When you scale an image up from the original scan the computer has to make up new pixels based on the information in the original scan. You do not get any more detail, just more data!
That is well and good, but how do I figure out what resolution to scan at when I need a 135% scan at 200dpi?
Simple mathematics will do the trick. Lets take the example you just mentioned. Right away we can tell you need more than 200dpi because you want a 135% enlargement (scaling) factor. First take 1.35 times 200 and you will get 270dpi as the needed scan resolution. But wait! That is an uneven scan frequency. The next higher even scan frequency for your 300dpi scanner would be 300dpi. Scan at that resolution and then scale the resulting digital image down to 270dpi making sure you are really making the file smaller (uncheck the constrain file size box in PhotoShop's Image Size dialog). You now have the correct size file, but the physical dimensions and dpi are not right yet. Go back to image size and this time make sure the constrain file size box is checked. Now change the dpi to the amount you needed to begin with...200dpi. Presto! You now have a 135% at 200dpi image that looks better than it would if you let the scanner do bilinear interpolation.
A good way to bypass all that math is to figure out what physical size and resolution you need by looking it up in a Handy Dandy Chart, or create a NEW file in your image editing software that is the correct size and resolution and look at the indicated file size. In your scanner interface there should be a place that shows the resulting file size for the scan you are about to do. Make sure the settings you are about to use for the scan will result in an EQUAL or LARGER file than you want to end up with. It's only confusing until you understand!
Now I know half of you are skeptical at this point! So try this. Scan a photograph at some odd scan frequency, say 147 dpi at 100% scaling factor. Sharpen the heck out of the image and zoom in at least 5:1 zoom factor in your imaging software. You should be able to see individual pixels AND you will see groups of pixels outlined by color or contrast changes. Larger cells made up of groups of pixels in fact. Now try it my way. Guess what? No groups. I first noticed this effect totally by accident after sharpening a photo of a person in profile. Their nose had stairsteps down it. After redoing it at an even scan frequency, the photo looked much better with no stairsteps down the nose.
Well that is it for this week. Next week we'll talk about how to quickly and easily color correct your color scans!
Thomas E. Seal
President & Chief Bottle-Washer
DigiMatrix Digital Darkroom
820 South Central Ave.
Medford, Oregon 97501
U.S.A.
(541) 734-2225 voice
(541) 734-2476 fax
email: digimatrix@wave.net