Resolution: measures how much detail is captured

The film scanning process must capture the smallest detail on the film.  The finest film that exists has “dye clouds” which can be as small as 6 microns in diameter.  

At 6,000 DPI, each dot that we image from the film is about 4 microns in diameter. Therefore, we can capture all the detail from the film, but further increases in resolution would not make a difference.

35mm Film Resolution for Sharpness and Focus:

When scanning film to digital format, higher resolution makes the scanned picture look sharp and in focus. Knowing that computer screens at best have 150 pixels per inch (ppi) resolution, while most have 72 ppi, people are confused. It may seem that if you scan your slides at 2,000 ppi they should look great on your computer. However, this is not true, and here is why:

• Your computer magnifies the images to put it on the screen.  A 35mm scanned slide or negative gets magnified 100 times to fill the screen on a notebook computer.
• In days gone by, people got up out of their chairs and got closer to the screen when they needed to see more detail when viewing projected slides.  Today they zoom in to meet the same need, increasing the slide scanning resolution requirements.
• We humans look for the edges in a scene.  Sharp edges appear to be in focus, whereas fuzzy edges appear to be out of focus.  If you zoom in a lot on an edge, you will see “digital artifacts”, the checkerboard edges of all digital images.  If you zoom back out, it looks like they disappear, but they don’t really.  The checkerboard is still there, but we don’t see it as a checkerboard anymore, we see it as a fuzzy edge, which looks out of focus.  The finer the checkerboard, the more in focus the image appears.

For these reasons, digital image quality improves exponentially with more resolution.  Film scanned at 6,000 DPI will look almost 10 times better than the same slide scanned at 2,000 DPI.

A Word About Metadata Resolution Numbers…

Metadata is “Data about Data”.  Every film scanning service embeds metadata in the image file when they scan the slide.  This is where information about the picture (like name, date taken, etc.) is stored. You can view this information by right clicking on an image in Explorer and clicking on “Properties”, or choosing “View Metadata” in a photo editing program.

There are hundreds of data items available. Three data elements which are relevant to resolution are resolution, width and height. Resolution values range from 72 to 9,600. To understand the resolution of the scanned slide or negative, a 35mm film scanning service must also look at the height or width of the image. For example, I have a scanned image of a 35mm slide. The physical dimensions of the slide are 1.37 x 0.90 inch.

The metadata is as follows:

Resolution: 72 dpi (dots per inch)
Width: 3145 pixels
Height: 2072 pixels

Do the math, and the picture size works out to 43 inches wide by 29 inches high. Digital cameras assign resolution numbers depending what the manufacturers think you are going to do with the image. A resolution of 72 dpi corresponds to computer monitors. Other cameras assign a resolution of 300 dpi, as this is what a lot of printers use.

Now, back to the 35mm slide or negative scan. The resolution that the image was made at can be calculated by dividing one of the image dimensions (in pixels) by the actual size of the 35mm slide. This reveals that the image was scanned at (2072 / 0.9) = 2300 dpi.