A few scanning tips

Now, explaining the optional second method, the use of the scanner's scaling control during the scan (the previous page described scaling existing images AFTER the scan).

The principles of this and the previous method are exactly the same, and it is the same simple arithmetic. The only difference is that the scanner's Scaling control is a calculator to help us with the arithmetic, and I will try to explain how it is used. In commercial practice, often the image has only one specific purpose, like to be printed in a particular magazine, with the size and dpi requirements being known exactly. For that kind of specific purpose, then the method described here can be of help with the arithmetic.

I usually instead do the arithmetic manually, as previously described instead. If I know I want a 3 inch printed image size at 300 dpi, then I just make sure I scan to create a 900 pixel image size so I can scale it. But that is not the only way.

Using the Scanner's Scaling Control

This may come as a surprise, but the resolution entered into the scanner software is NOT the scanning resolution. We actually enter the printing resolution there.

If the scaling factor is 100%, then the scanning resolution is 100% of the printing resolution, therefore they are the same numerically, and it is OK to think of it as the scanning resolution. If we scan at 300 dpi at 100%, it scans at 300 dpi x 100% = 300 dpi. But this equality is only true at 100% scaling. At 100%, the image is also scaled to print at 300 dpi, and that is the main idea.

If we change the scale factor to say 50%, then the specified 300 dpi resolution is still the printing resolution. However, the scanner will now scan at 50% of that 300 dpi value, or at 150 dpi scanning resolution, to create the right number of pixels to print half size at 300 dpi. The indicated Output size in inches at 50% is half size of the original. That output image will be scaled to print at the specified 300 dpi, we asked for that. If you change the scaling away from 100% for printing, then this effect is what you will see, how it works.

So, if it is desired to scale the image's printed size during the scan, do NOT change the scanning resolution setting like you would to change size on the screen. It does not work for printing, you'll always get the same printed size at 100%. To use the calculator, leave the scanner's resolution setting at 200 or 300 dpi, whatever is appropriate for your printer, and instead use the Output Scaling field to modify the output size. Some film scanners call this field Magnification, enlarging the film size by perhaps 1000% or 2000% when printed, but flatbed numbers are normally smaller, maybe 50% or 200% (half size or double size). The scan resolution will be calculated and the image scaled to this size change at the specified printing resolution for the printer.

This Microtek Setting Window shows we are scanning a 4x4 inch photo area, and scaling by 200% to print double size as 8x8 inches at 150 dpi. We know the photo's Input area we want to scan, so we set that, probably with the mouse on the preview window. If we want to print at 150 dpi, we set that here for our printer. We know we want the printed image to be 8x8 inches, so we specify a percent scaling factor to get that size. We simply entered the number 200% in the scale factor field to get the desired output size in inches. This 200% then causes the scanner to double the resolution, to scan at 150 dpi x 200% = 300 dpi. This scale doubles the size of the output image, to have pixels enough to enlarge 4x4 inches to 8x8 inches at 150 dpi.

There is a risk. If we don't pay attention, we may not be aware of the actual scanning resolution when scaling. The actual calculated scanning resolution is not shown (on most scanners), and we might exceed the scanner's optical rating without realizing it. We can happily set the resolution to 300 dpi and the scaling to 300% to get the larger image size. The scanner scans at 900 dpi in that case, but we might have a 600 dpi scanner, so it must interpolate to do what we asked. No hand comes out and slaps us, but we exceeded the optical capability of the scanner. Interpolated results won't be as sharp as we expect. We don't have any warning of this, we just have to be careful and realize how it works.

Another factor, this method can end up computing an odd scanning value like 367 dpi, or a similar non-standard value (see integer divisors).

This is the difference and meaning of scanner Input and Output. Input is what we scan, and Output is what we create or print.

Some drivers show the scaled resolution, for example, the Minolta Dual II at right, specifies both the scanning and the printing resolutions, called Input and Output. Shown is scanning 35 mm film at 2820 dpi, and will mark the output file to print 16.1 x 10.9 inches at 240 dpi. That's very clear, it will always scan at the Input dpi value at 100% scale (no other scale menu). Output is the scaled printing resolution, and it shows the printed size in inches (you can always scale it differently later, but if you specify 240 dpi, that's what you get).

We think of scaling being for printing and not for the screen, but since scaling affects the scanning resolution, and the number of pixels created, it affects size of an image on the video screen too (screens show pixels). The difference is that for the video screen, you can get the same double pixel size by doubling either the resolution or the scaling. If we look at Output size in pixel units, specifying 4x4 inches at 300 dpi at 100% scale creates the same 1200x1200 pixels as does 150 dpi at 200% scale. The images will look the same on the screen. But the 300 dpi 100% image is scaled to 300 dpi, and prints original size at 100%. Specifying 150 dpi at 200% scale also scans at 300 dpi to create the same pixels, but it is scaled to print at 150 dpi, which will increase the printed image size to double size (200%).

We can scale manually, or it may be easier to have the scanner driver take care of the math, using the scanner's scaling control. We can simply set resolution to 250 dpi (whatever value we use for printing), and set the Output Scaling to some percentage size to create the desired output size. This 200% scaling creates an image that is larger in inches, pixels, and resolution (beyond what was specified). The scanner driver will know what we're asking, and will do the calculations to create it.

The chart below illustrates the two ways to double the image size in pixels of a 150 dpi 4x4 inch scan. We can specify 300 dpi 100%, or we can specify 150 dpi with 200% scaling. Both methods display the same size on the monitor, but only the second way will print at 200% size.

Now, don't make this difficult just because I am. It really is pretty easy, and the point belabored here is that the scanner does exactly the same thing either way. Either way, the scanner actually scans at 300 dpi and creates the same 1200x1200 pixel image from a 4x4 inch input area. That is the scanning resolution actually used. Conversely, the output resolution number is just a note or comment that is carried along with the image. This is the scaled resolution, or the printing resolution, and it is just a number, a numerical value, nothing more. It only matters to the printing software, to tell it how to size the image pixels on that output device later. The lesser resolution number will print larger, because 150 dpi will print dots (pixels) twice as far apart than does 300 dpi (the centers of the pixels are twice as far apart, therefore the pixel's size is twice as large). 300 dpi dots are closer together than 150 dots to the inch, so the same 1200x1200 pixel image prints smaller at 300 dpi than at 150 dpi.

So there are two concepts of resolution when scanning for the printer. I'll call them scanned resolution and scaled resolution.

Scanned resolution is what creates the actual image size. The original has let's say an area of 4 by 4 inches, that was scanned at some specific dpi resolution, say 300 dpi at 100%, which created an image that is X by Y pixels now, 1200x1200 pixels in this case.

When we print this image, the drivers print the pixels on the paper at the spacing determined by the scaled resolution, that is, 300 dots to the inch in this case, if it was not changed, and if it was 100% scale factor. So the printed image size comes out as 1200 pixels / 300 dpi = 4 inches. Our image is printed the same size as the original, 4x4 inches (because scaled and scanned resolution are equal at 100%).

If we had originally scanned 4x4 inches at 150 dpi 100%, we get 600x600 pixels, and printing at 150 dpi gives the same 600 pixels / 150 dpi = 4 inches, the original size again (100%) but with lower printed resolution.

So changing scanned resolution will not change the printed image size. But changing scaled resolution certainly will, that is its purpose.

Scaled resolution is what I am calling that resolution number that we enter for the scanner (seems backwards, but it's not). This number is used to determine the pixel spacing on paper when printed, and therefore determines the actual size of the printed image. We can scale the resolution when we scan, which increases the scan resolution so when we print it twice as large perhaps, we'll have plenty of pixels. Scanned resolution is that entered number multiplied by the specified Scale factor, most often 100%, and is the dpi resolution of the scanner samples.

A 100% scale factor means the scanned and scaled resolutions are the same. Therefore our scans will print at the original size (if our printing software doesn't meddle with its own notions). But if specifying 150 dpi with 200% scaling, it doubles the actual scanned resolution. Twice more is used (300 dpi) than is specified, which makes the image larger, in the sense that 1200x1200 pixels is larger than 600x600 pixels. So 300 dpi is the scanned resolution, what the scanner actually does, and 150 dpi is scaled, how it will print. The point is that the remembered value of resolution (for printing later) is the specified value, 150 dpi. This is the scaled resolution that makes the printed image print twice as large, large in the sense of inches.

We can always scale at any later time after scanning, assuming we originally scanned the image large enough to have sufficient pixels available to get a decent printed image. If we were to print this 300 dpi image above (the same exact 1200x1200 pixel array) later scaled to 150 dpi instead, the printed image would be 8x8 inches, double size. That is called scaling too, we have scaled the image to 200%. It prints 200% larger, because the same pixels are printed farther apart, only 150 to the inch instead of 300 to the inch. This way is my preference, it seems easier to take the direct route, probably just a matter of having full control of all details.

There is no particular magic or mumbo-jumbo in these dpi numbers and scaling factor. One is the scanning resolution and one is the printing resolution, and at 100% scale, they are the same. What we enter and see is the printing resolution.

One should keep in mind that scaling is intended for printing. The scaled dpi resolution of an image has no meaning to the video drivers, it is ignored. Only the image dimensions in pixels has any meaning to the video monitor