Actually, I was wondering if anyone had any guidelines for scanning negatives? For example, if I wanted to scan 35 mm negatives for enlarging to 4×5, 5×7, etc. (to print) are there any guidelines for what resolutions to use? Or, does anyone know where I can look?
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Any time you scan at a resolution that resolves smaller than the grain of the film you run the risk of grain being overemphasized. The resolution that resolves the grain obviously changes with the types of film. Color negative films have a grain size in the 13 – 16 micron range, making optimum scan resolutions somewhere in the 1600 -2000 ppi range. Color transparencies have much finer grain, with the newest films from Fuji being in the 4-5 micron range. Those films need something on the order of 6000 ppi to fully resolve the grain.
That being said, scanning at a resolution that clearly resolves the grain of the film doesn’t necessarily mean a scan with more detail than a lower res scan, as the image itself might not contain any usable detail that fine, possibly due to limitations of lenses being used or even camera movement.
Most CCD type scanners have only one true optical resolution – the max that is claimed. When you ask the software for less than that number, the scanner is still scanning at max and the file is being downsampled in the scanning software. Drum scanners, on the other hand, have multiple resolutions defined by both the speed of the drum passing in front of the lens and by a set of apertures behind the lens, defining the "slice" that the lens sees per drum revolution. By matching the aperture to the grain size of the film, the scanner can be set to scan whatever file size requested but yet not cause a grainy mess at the same time.
Peter Figen – wrote "Most CCD type scanners have only one true optical resolution – the max that is claimed. When you ask the software for less than that number, the scanner is still scanning at max and the file is being downsampled in the scanning software."
Are you sure about that? I am only asking because with my Canon scanner a 150 DPI scan is much faster than a 600DPI scan. I don’t claim to know if the scanner is scanning at full resolution and then down-sampling or not it just seems odd that a scan that, in your scenario, would require more processor power is faster.
The scans are faster because your scanner is probably looking at every 4th pixel.
Nearly all negative/print scanners user a linear array, and a stepper motor to advance the film one line at a time. The number of sensors on the array cannot change, but the number of them that are used may. It’s better to use them all and downsample the result using some filter, but it sounds like this is not happening with your scanner.
Depending on the type of array used, the R,G,B pixels will probably not be colocated, and only if you use alternate resolutions you will be lucky if the stepper increment coincides with the distance between the R,G, and B sensors. What this means is that a given CCD-stepper combination is only good for certain image resolutions (the stepper increment must be an integer fraction of the distance between the R,G,B sensors) Good scanners will make sure that this is the case. If it isn’t you will end up with color planes that are slightly offset. This can result in fringing at high magnifications. There are filters to correct this- (Catmull-rom, I think) but that’s just more processing.
The point is, get a good scanner, and scan at the default/optical resolution.
