How to do that? Very simple. The term dpi means Dots Per Inch, referring to image "pixels per inch" (dpi and ppi are the exact same thing as related to images, but printer rating dpi is different than image dpi, see the Printer Basics section).
Assuming 100% scaling (see Scaling), the meaning of scanned dpi is that if you scan a 6x4 inch photo at 110 dpi, then you will necessarily get an image size of
(6 inches x 110 dpi) x (4 inches x 110 dpi) = 660 x 440 pixels
which more or less totally fills a 640x480 monitor screen.
Or scanning the 6x4 inch photo at 140 dpi gives
(6 inches x 140 dpi) x (4 inches x 140 dpi) = 840 x 560 pixels
which more or less totally fills a 800x600 monitor screen.
Or scanning the 6x4 inch photo at 180 dpi gives
(6 inches x 180 dpi) x (4 inches x 180 dpi) = 1080 x 720 pixels
which more or less totally fills a 1024x768 monitor screen.
We are not being very fussy about the exact screen dimensions, but you know the size of the area you are scanning (inches), and you know the size of the video image you wish to achieve (overall pixels), so you adjust resolution to get it (dots per inch, meaning pixels per inch). It depends on how many inches you have, and how many pixels you want. That's how it works, that's all there is to it. Really!
The idea is like this: Those three scanning resolutions just mentioned would create three different sized images from that one photo that could be shown on three screen resolutions, something like this:
In real world practice, we are much more likely to scan at round numbers like 100, 150 or 200 dpi (giving 600x400 pixels, 900x600 pixels and 1200x800 pixels, from a 6x4 inch photo) instead of 110, 140, or 180 dpi (see about integer divisors), and these may be the only choices the scanner offers anyway. Regardless, the image size you create (pixels) is computed from the inches scanned and the resolution used, as shown above. The image size you want depends on your purpose, how the image is to be used. You scan at whatever resolution is required to create this desired image size from the inches you have to scan. If the image is for the screen, then the computer video system will show those pixels one for one, at that size on the screen. It is as simple as that.
So what is important to determine scanning resolution to create a certain image size is:
And the obvious answer is to select a resolution that will scale that input size in inches to that desired output size in pixels.
You have a 4x6 inch photo portrait, and maybe you want the six inch dimension to fill a 800x600 pixel screen vertically. Then obviously 600 pixels / 6 inches = 100 dpi scan resolution is required. Exactly 100 dpi, no more, no less. If the overwhelming requirement is that it must fit the screen, a different resolution is simply not a consideration in this case. Scanning 6x4 inches at 100 dpi will produce an image of (6x100) x (4x100) = 600x400 pixels, aligned vertically just fills the 800x600 pixel screen height. Horizontally, this 400 pixel image would fill half the 800 pixel screen width, so we could put two of these side by side. However, if it were a 8x10 inch photo, then 600 / 10 = 60 dpi resolution would produce an 480x600 pixel image, which fits vertically, but we would have to crop to fit two of them horizontally.
The point is, we can easily predict the exact results by just looking at the numbers.
The scanner's twain driver will measure and calculate this output image size for us before the scan. Just change the settings to show pixels instead of inches and it will show the final image size corresponding to the cropped Input area in the Preview, which corresponds to the resolution you have specified. Both the Input and Output fields change to inch or pixel units.
This example scans 6x4
inches of photo at 100 dpi, and creates 600x400 pixels (100% scale is
assumed - see Scaling). Or going the other
way, if you want a specific image size, you can type that specific size
into the ScanWizard Input window, and the Preview area will change to
that size, which you can then center on your image to crop as desired.
This is Microtek ScanWizard 5, but regardless of which scanner, the idea
is always the same. You specify a resolution, and the size of input image
(typically by selecting an area with the mouse in the Preview), and these
numbers create the specific size of the output image.
So for an image which is to be shown on a video monitor, you choose scanning resolution to get the desired image size. If your purpose was to show it on a 800x600 pixel screen, then you would NOT scan at 300 dpi to get a (6x300) x (4x300) = 1800x1200 pixel image. Images can be resized, but over ¾ of your total pixels would have to be discarded to fit the screen (what a waste!).