I'm often asked how to determine the optimum scanning resolution for large format film. (All smaller formats are scanned at maximum meaningful resolution of 4000 dpi.) The answer depends on your expected maximum print size and on the printing method to be used.
Let's say that you want to make a 32" x 40" print from 4x5 format. If you're using an Epson printer, its native printing resolution is 360 dpi. (Other printers, such as Lightjet and Chromira, use 300 dpi, or close to that figure.)*** Multiply the long side of the desired maximum print size (40") by printing resolution (360 dpi)...this results in 14400 pixels needed to create a print of that size without adding upward interpolation. While one can perform the latter operation to a certain extent, the very best results will be obtained without adding significant interpolation. A small amount of upward interpolation, such as 5% or 10%, will probably be unnoticeable in the final print.
Divide this resulting figure, 14400, by the long side of the film format, 5 (inches). Although the actual image area is slightly less than 5 inches due to sheet film's margins, it's close enough to use for our figures. The result is 2880. This means that you can scan at 2800 dpi and add a slight amount of upward interpolation, or scan at the next higher step, 3200 dpi, and downsize the file to obtain the actual printing size. (Scanning resolution is only available in the steps listed on my drum scan price chart; for example, you cannot obtain a scan at 2932 dpi.)
Let's figure a second example in reverse order, i.e. to determine the largest print obtainable from a scan of xxxx resolution. If you are considering a 2000 dpi scan from 4x5, for example, simply multiple 2000 x 5 (inches), resulting in a final image size of 10000 pixels on the long side. (Remember that the actual result will be slightly less due to film margins.) To determine the largest print you can obtain from that 2000 dpi scan, divide this 10000 pixel figure by 360 (for Epson printers) or by 300 (for many other printers). The result indicates that the maximum long side of your print comes out to 27.78" without adding interpolation (Epson printers), or 33.33" without adding interpolation (many other printers). If you want a larger print, you'll need to scan at a higher resolution, or add a reasonable amount of interpolation. Keep in mind that adding interpolation will degrade image quality, so keep interpolation at a moderate amount.
***Important! Use your printer's actual native resolution to obtain these figures. Do not figure the needed resolution at, say, 200 or 300 dpi for an Epson printer that requires 360 dpi. The printer will add interpolation to obtain the 360 dpi that it requires, producing a print of lesser quality. While this may be fine for prints to be viewed at a distance, it won't do for "up close" viewing of a fine print. You also run the risk of applying sharpening at an unsuitable amount, since the printer is adding its own interpolation to an image that you have already sharpened at lower resolution.
The short answer - 2400 and 2800 dpi scans are my most popular from the 4x5 format.