so if a sensor is right where two film grains meet with very different colors or values, what does the pixel do? Does it choose one color or the other, or does it appear as an everage between the two?
rg2 On 9/19/07, Doug Franklin <[EMAIL PROTECTED]> wrote: > Rebekah wrote: > > Thanks Doug, I think you've made me even more nervous about having my > > slides scanned now! No really, that was all good to know. I'm going > > to read about GA like you suggested. Maybe I'll just have them > > scanned at 3000dpi, or better yet, just wait until I get my own > > scanner :-) > > Well, I didn't intend to scare you off. I intended something more along > the lines of letting you know what /could/ happen ... not leave you with > the impression that it would or probably would happen. I'm sorry for > doing that. It occasionally happens, and when it does, it's almost > always on a photo you really want to salvage. > > But it's not all that common, viewed in the large. It happens mostly > with particular films scanned with particular scanners. Avoid those > combinations, and your pretty much golden. > > GA also seems to be more common with consumer grade equipment. I > haven't heard much in the way of whining about GA happening on > professionally done scans. Maybe it happens but they've got more > adjustments or better technology to take care of it when it does, so the > customer never sees it. > > The rest of this is just sort of FYI explaining what causes it ... > > GA is a fact of life at certain resolutions. Basically, if the average > size of a film "grain" (actually "dye cloud" on color films and > chromogenic B+W films) is "similar" to the size of the sensor's pixels, > things can get colorifically strange when the grains and the pixel wells > overlap each other in some ways. > > Actually, it's when the size of the image of the dye cloud projected > onto the sensor is similar in size to the sensor's pixel. Similar in > this case seems to be somewhere around plus/minus half an order of > magnitude. > > You can get similar issues when making any medium transfer, including > copying film to film. It's just that on both films, the grains are more > or less randomly distributed, both spatially and in size. Which means > that the "erroneous" grains are also more or less randomly distributed. > This "takes the sting" out of it for the human visual system, and it's > much less noticeable. > > OTOH, the pixels on a sensor are rigidly and regularly distributed and > sized. The "erroneous" pixels are still more or less randomly > distributed, but somewhat less randomly than above. And they're less > randomly distributed by being on a harshly rectilinear grid of fixed > size features. That makes the effects stand out to human eyes when it > does occur. > > -- > Thanks, > DougF (KG4LMZ) > > -- > PDML Pentax-Discuss Mail List > PDML@pdml.net > http://pdml.net/mailman/listinfo/pdml_pdml.net > -- "the subject of a photograph is far less important than its composition" -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net
