The calculator is hopefully self-explanatory, but here are some added details.
Basically, it computes the answers to these questions:
Plus it tries to show by example how things work.
You select a desired size to be printed, and then on the right hand side, you select a film or photo size to be scanned. Then select EITHER a desired printing resolution to print it, or a desired scanning resolution, and then the calculator computes the other number.
Perhaps you want to scan 6x4 inches and print it in a document at 3x2 inches. Or maybe you want want to scan 35 mm film and print A4 size (near 8x12 inches).
Or maybe you want to know the maximum capability of a 4000 dpi film scanner if scanning APS or 16 mm film.
Or if you already have an image of known size (pixels), the lower left hand side shows the resulting printing resolution to give the selected print size.
Nonsense answers are not prohibited. For example, the calculator doesn't know that it is not practical to enlarge 8mm film to print 11x14 inches at 300 dpi. It will dutifully compute that this requires 25,400 dpi scanning resolution. <grin> This is not practical of course, but you will know your capabilities.
Desired Output Print Size
This is the size of image you want to print, which is required for all calculations. Select one dimension, and then specify if it is the SHORT dimension or the LONG dimension. This Short or Long dimension will be held fixed (won't vary), and the other dimension will float to fit the image shape.
Either 8 inch Short, or 10 inch Long can specify a 8x10 inch paper size. But these may give different results, for example, if scanning from say 35 mm film (with aspect ratio 3:2), 8 inch Short may come out 8x12 inches (must crop the long end). But specifying 10 inch Long might come out 6.67 x 10 inches (short dimension may underfill and must be trimmed), simply because this is the shape of the film. These numerical results will be shown.
But the point is that the specified dimension (short or long) will be fixed (won't change), and the other dimension will float to come out whatever it must according to the shape of the Input size (shape of the scanned film or print size).
This method gives you more control to specify the desired print size - the same questions you must answer when actually printing on paper.
Printers generally cannot print to the paper edge - you cannot print an 8.5x11 inch image on 8.5x11 inch paper. Printers have required nonprintable margins. Your printer's specfication will specify its maximum printable area for various paper sizes.
If your printer is going to leave a margin all around, you can plan for it by computing the margin size too. The actual printed image will be computed a bit smaller to allow for the margin, so that the entire image is inside that margin.
Existing Image Size
If you already have an image of known size (pixels), for example digital camera images, this will calculate the printing resolution possible to print a given print size (which you specify at the top). A few common or interesting sizes are offered, or you can choose CUSTOM VALUES and enter any numbers for width x height (pixels).
Selected Input Size, Film or Print
If you are scanning, this is the size of film or print or document that you are scanning. For example, if you are scanning 35 mm film, it is 36x24 mm in size. If you are scanning a 6x4 inch photo, this tells the calculator the size of it. There are many standard sizes offered, including older film sizes.
We cannot often scan the entire frame size. We don't want the edges to show, and slight skew can show that even worse. If you want to crop to a smaller frame, you can specify a margin size all around, to be omitted from the scan. This is simple cropping. This Crop Margin makes the scanned area smaller by this dimension, on all four sides of it (equal all around).
For example, a 1.0 mm margin around mounted 35 mm slides is often a good thing to do, resulting in 34x22 mm film area instead of 36x24 mm. You can specify the units for this margin in inches or mm (like 0.125 inch or 4.5 mm). You will see an error message if the margin is so large there is nothing left inside it.
If you know you want to crop much more, perhaps an inch all around a full page document, you can specify that wider inch margin to crop too, as 1.0 inch. Then the scan area for 8.5x11 inches with a 1 inch margin will come out 6.5x9 inches. This may be a way to specify a scan size that is not in the list. It varies the input size to be computed.
The margin can be negative too (like -0.5 inch), in which case the size of the scan area (film or print) is increased instead of reduced, and it will compute the scan size including a border area outside the frame. This may be useful if you are scanning film or print size not in the list. 4x5 inches with a -0.5 inch margin all around (a negative margin) is larger, 5x6 inches.
To Print at xxxx Resolution
This is is the desired printing resolution, and it will compute the corresponding necessary scanning resolution (to print this print size from this input size). If you want to sufficient pixels to print the image at 300 dpi for example, this option will show the necessary scanning resolution that will do that. Generally you want to print photo images at 250 to 300 dpi.
To Scan at xxxx Resolution
Given this specified scanning resoluton, you can compute the possible printing resolution that will print this print size from the pixels created by this input size and this scan resolution.