For printing, the image resolution is expressed in dpi. A resolution of 100 dpi means that 1 inch of paper width will depict 100 picture elements. This is a relative resolution, since the number of pixels relates to the physical print size. It can be changed at will without quality loss (just make sure to uncheck the option “Resample Image” in Photoshop, which by default is on). More important is the absolute resolution, which is expressed in pixels. For instance, reducing the absolute resolution will result in loss of detail and should never be done in the digital negative.

The absolute resolution of a desktop wallpaper might be 1024 × 768 pixels. From the absolute resolution and the relative resolution, the output size in inches can be calculated. Most viewers and editors calculate this automatically for you. In Adobe Photoshop, you can define each parameter separately, and the program will adjust the other parameters accordingly.

The output size of an image depends on the resolution of the output device. In our case the output devices are monitors, home printers, or the photo lab printer. To stay with the example of desktop wallpaper: if you display it on a 17″ and a 19″ LCD monitor, and both are set to a resolution of 1024 × 768, then you’ll get one image displayed with a 17″ diagonal and the other with a 19″ diagonal. As we can see, the output size depends on the output device, or rather on its relative resolution. Depending on the chosen output device, the image requires different minimum relative resolutions in order to look sharp. Magazines, books, and prints from the lab use a print resolution of 300 dpi; inkjet printers use 200–300 dpi. Considerably lower values result in soft images.

Once you have bought a scanner, you’ll want to make prints from the image files either with your own printer or with the help of a service provider. You’ll want to know how far you can push the quality of your scanner. The size of the scanned file (in pixels) depends on its optical resolution (in spi, or as is more commonly used, in dpi).

From that you can calculate the maximum print size in inches. It is no problem technically to down-sample a large file, but it is not possible the other way around. If you have a 4000 dpi scan and scale it to 8000 dpi afterwards, you will not produce more picture information than was in the original scan. The only effect is that you waste a remarkable amount of storage space, as the 8000 dpi file is twice as large as the 4000 dpi file. Therefore, scanning is the key process that determines the quality of the image that can be achieved.

Always keep in mind, that the bottleneck in scanning resolution is in most cases the quality of the slide and not the ability of the scanner!

You can calculate the maximum print size quite easily yourself as shown in the following three steps.

A 72 dpi resolution is sufficient for viewing images on a monitor, but you will need more pixels per inch for printing. A book or magazine image only appears really sharp when it is printed at a resolution of 300 dpi or more.

Here is a very simple trick to improve image quality: set the scan resolution to the optical resolution, e.g., 4000 dpi, and the scale to 50 percent (= 2000 dpi). This effect cannot be reproduced afterwards in an image editor; it has to be done during scanning. It is due to the effect that scanning in full resolution produces more noise, than scanning with a lower resolution. If you just scale down the scan afterwards, the noise is still there; at least in subjective matter, it gets even more visible through scaling. The best practice is to scan the slide at the same resolution you want to use later. If you scan primarily for archiving purposes, 2000 dpi is a good compromise.