DPI, or dots per inch, is a way of measuring the density of the resolution of a scanned photo, slide, or negative. A dot, or pixel, is a single unit representing one color in a digital image. Many pixels together make an image on your screen. Scanning at higher resolutions divides the same picture into more pixels.
DPI varies by media. Most photographs, printed at 1-hour photo huts, have about 600 DPI worth of resolution. This means you will get an increase in image quality going up to 600 DPI, after that you reach the physical resolution the media you’re scanning. For slide film and negatives, 2000 DPI – 4000 DPI is appropriate to capture near the limit of detail rendered in the celluloid emulsion.
A 35mm kodachrome slide image boundary is 34m x 23mm, converted to inches because it’s dot per INCH, 1.34” x 0.90”. At 4000 DPI, the image scanned will be 5360×3600 pixels or close to 19MP. There are 2 factors limiting the resolution captured on film to around 20 megapixels; film grain, and lens quality. Similar to pixels, film is made of small light sensitive particles. In color film these particles measure about 10-25 microns. So the “physical resolution” of film is about 2500 DPI.
Cheap lenses can reduce the detail in an original image, blurring it considerably. Low quality glass or plastic lenses on many cameras from the 70’s, 80’s, and 90s limits the quality of many images from this time period to 2500 DPI or less.