Archived posting to the Leica Users Group, 2001/03/07
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Off the top of my head, I think you are seeing the result of the Nyquist limit. The scanner's scan frequency is very close to the film grain distribution frequency. This causes a moire, a jaggie, an uneven clumping, and many other not wanted effects. And, of course, the grainer the film, the easier it is to get these effects with consumer low res scanners. A very hi res drum scanner will not have these problems since the scan frequency is thousands of times higher than the grain distribution frequency. So each piece of grain is scanned at a thousand intervals and the space before it is scanned at a thousand intervals, as is the space after it. So there is a plethora of information and when reassembled, accurately represents the original. A low res scanner will possibly hit a space, miss the grain, hit a space, miss a grain, etc, producing a large void, and as frequencies beat, the scan will hit a grain, miss a void, hit a grain, miss a void, etc, causing a large density area. This will look like clumping and turn your Pan F in Xtol negative into a Dealt 3200 in Rodinal print. Color doesn't get off scot-free. Kodachrome is B&W film and has grain. E6 has color globules and these can be disbursed at a frequency to cause color clumping, color moire, color jaggies, etc... The same problem exists in digital camera sensors as the pixels are evenly spaced and the spacing frequency may limit the lens resolution. Too much resolution will cause these problems since the pixels will be missing information at an exact frequency, therefore dropping out evenly spaced picture elements. OK, I'll stop now... Jim At 01:54 PM 3/7/01 -0800, Richard Edwards wrote: > >If you saw prints of these, you'd see similar sized chunks of >grain, but more subdued, random, and pleasing to look at. > >The oatmeal in the shadows is grain aliasing. Scans >are good, IMO, but the pixels are having trouble >stumbling over the boundaries of the grain. Life isn't >fair: The scanner has enough resolution to do the >negative justice, but details below the level >necessary for our eyes don't mate with the hardware. > >Pixel size is close to grain size, so interference >patterns result, as with a herringbone >tweed jacket on television. A scanner hardware >issue -- they all do it. > >In the future, when we get to a zone where >pixels are several to a grain, this problem will >diminish to a second or third-order concern, or so >I'm told, and we'll no longer have to worry about it. > >Mostly a problem with black and white. Check out: > >http://www.photoscientia.co.uk/Grain.htm > > >-A L