For slide films (the finest color films available), the grains are 7 to 8 microns in typical size. Technically, you need to sample the scan with pixels that are 7 microns in size, meaning 3600 dpi, to gather all the information from the film. If you scan with higher resolution (available in modern film and drum scanners), you detail the grains themselves, which may be important for the aesthetic result of the scan, but it does not bring more information into your scan. Practical and subjective tests suggest that for top-quality low speed films, sampling resolution can be about 4000 DPI (or around 5600 pixels). For a 3:2 frame of a 35mm film, that means around 20M pixels. Of course we're talking about ideal shooting conditions: on a tripod, in decent light, mirror-up, with a top-rate lens and the finest-grained film.
So the key question here is to what resolution can your film be scanned before the film itself and its granularity becomes the critical factor. Another problem is the optical resolution of your scanner. Only high-end film and flatbed scanners produce good results at their full optical resolution (being as high as 5400 ppi for modern flatbed scanners). Trying to get too much from your scanner will most likely produce blurry, fuzzy scans that are exaggerated in color distortion, that will be unacceptable to for collection.
For the highest quality scans there is no substitute for the drum scan. This specific type of method is a varied process from average desktop scanners, by working with transparencies and negatives against an acrylic drum to capture the finest detail at high resolution from the original image. A good drum scanner has an optical resolution of 12000 ppi, which is way beyond the resolution of media being scanned.
Let’s see some examples of negative films scanned beyond the optimal optical resolution of film scanner. Look at the fuzziness, lack of fine details and poor focus apparent in these scans:
These examples show other problems you may encounter while scanning your photos:
Emulsion defects may look like dark spots and should be retouched digitally.
The black borders of a film frame or, more often, black corners may appear in your scans. Frames are never perfectly rectangular in shape and have rounded corners. You can fix this digitally by cropping or retouching the image.
“Newton's rings” are an interference pattern caused by the reflection of light between two surfaces – a flat surface adjacent to a spherical surface. In monochromatic light this phenomenon appears as a series of concentric rings that alternate between dark and light. Viewed with white light, it forms a concentric ring pattern of rainbow colors. Newton rings are encountered in flatbed scanners and old fashioned film scanners with glass film holder. To get rid of them you can use Anti-Newton Glass or fluid scanning technique.
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8.0 Model & Property Releases