I am about to scan photo negatives with VueScan from Hamrick Software. But I don´t want to simply convert the negatives to positives and store them as downscaled and heavily compressed JPEGs, I first want to save high-quality digital equivalents to the analog negatives in the way described at http://www.colorneg.com/scanning_slides ... re/VueScan, building a kind of "digital archive", mainly because digital algorithms e.g. for dust removal and (automatic) color correction become better from year to year while the quality of the original negatives degrade at the same time. I don´t want to do any (heavy) post-processing on this archive. The images are saved as "RAW TIFFs" (RGB and a fourth alpha channel that contains dust and scratches information taken from an infrared scan; 64 bit = 16 bit for each channel).
Inverting the colors, removing dust and scratches, maybe even further scaling should be done in a second step - and could be repeated later easily using batch processing if any of the algorithm gets better over the years.
Even though I still have some problems opening the generated RAW TIFF images with ImageMagick (see viewtopic.php?f=3&t=22037), I am facing the problem that the images are too big - without any benefit. Every image currently has a file size of ~125 MByte with an image width of around 5000 pixels. The reason why I use this high scanning resolution can be found e.g. here: http://www.filmscanner.info/en/PlustekO ... 7600i.html:
I compared a scan with 7200dpi against a scan with 3600dpi and did nearly see no improvements in details (at least on my photos), so I stayed with 3600dpi in the scan settings (I don´t believe in the mentioned 2600dpi - but this is a different story). Using lower scan settings clearly showed a decrease in details.If a scanner with a nominal resolution of 7200 dpi provides in practice a resolution of only 3250 dpi, then it is obvious to scan with half of the resolution of 3600 dpi. But a resolution test with 3600 dpi resolution resulted in only 2600 dpi of effective resolution provided by the scanner, thus approximately 20% less than in the case of a scan of 7200 dpi...
But if I want to get the "real" resolution and compare the resulting 3600dpi scan against scales (done in Photoshop) with the resolutions of 8 and 10 Mpix digital cameras (3456x2304px/3872x2592px), I believe to see a loss in details in 8 Mpix resolution, while 10 Mpix seems to be ok. If I scale to this size of 3872x2592 pixels with Photoshop, I get a file size of around 75 MByte/image - which is far better and IMHO no more waste of space.
But then I face the next problem: Which is the best downscaling algorithm for this task? Photoshop´s bilinear and bicubic are known to be outdated, that´s one of the reasons why I want to use ImageMagick.
I have read the posts in this forum about downscaling and the impressive summary at http://www.imagemagick.org/Usage/filter/nicolas, but in my case, there is a different situation: I don´t have the colorspace of positive images and not the sharpness of digital images, but of course, I also don´t want Moiré (I don´t expect this at this level of details but you never know) and want to avoid any sharpening/ringing artifacts at this "archive" stage. Especially the alpha channel that carries the dust and scratches information (infrared channel) should not have any ringing artifacts because it is the base for future dust and scratch removal algorithms.
Do you have any recommendations, ideas or experiences on this special case?
Thanks a lot in advance!