Dots per inch (DPI) is an important term to know when it comes to not only film scanning, but digital photography and digital editing in general. DPI is used to describe the resolution of dots per inch in the print resolution of a hard copy print. As the DPI increases so will the final print quality of your photograph. Similarly, as the DPI decreases the final print quality will equally suffer. 300 DPI is typically the recommended print resolution. However, it should be known that inkjet printers typically range from 300-600 DPI while a laser printer may range from 600-1,800 DPI. Along these lines, the higher the DPI of an image, the more space it will take up on a computer hard drive. With the affordability of external hard drives it may be best to simply scan film at a higher resolution than you may need simply so you have it, just in case.
When scanning film, particularly 35mm film, the DPI of the scan needs to be fairly high so that a small negative frame can eventually be printed in a larger size. In general, 35mm film should be scanned in at least 2,400 DPI – which results in a final print of 8″ x 10″ at 300 DPI.
It is important to know that while you can scale a high-resolution file down to a lower resolution, you cannot scale a low-resolution file up. Attempting to make a 72 dpi image 300 dpi will “work” as you see it on screen but will not work for a final print. If you have a large dimension photo at 72 dpi you can convert it to 300 dpi, but this will reduce the dimensions of the printed photo.
While it ultimately does not matter, you will often hear that you should use 72 DPI for a computer screen. 72 DPI is a false notion but has managed to become a standard in the image editing world. In fact, DPI has absolutely no affect on a computer monitor, instead the focus needs to be on the pixels. For example, here are 3 images at varying dots per inch. Each image is 500 pixels wide by 332 pixels high, but one image is 1 DPI, one is 72 DPI and one is 300 DPI. Can you tell the difference between them?
The difference between these images will only be seen when attempting to print them. As previously mentioned, ultimately this fact does not matter as most people will request low resolution digital images at 72 DPI. It’s simply something to help understand digital imagery versus printed photos.