Colour fringing with V750?

[walter23]

Hey, good idea. I'll do that.

[Ben R]

Can I understand that it is better to scan as RBG eventhough you will have misalignment of the channels, because you can then use only one channel which will offset any of the misalignment problems. Would it not be better to scan as B&W in the first place or is the RGB channel method using 1 of the channels still better? Do you not loose any of the tonality by doing that?

[Michael Mutmansky]

Ben,

We're talking B&W images here, so there is no useful color information in the three separate channels, and in most cases, the three channels are mostly duplicates of each other.

That said, due to the color fringing problem, scanning in B&W mode will almost always be less sharp than scanning in RGB and then dropping the R and B channels in PS. The scanner software essentially takes the R, G, and B channels and makes a gray value equal to the average of the three. That will lead to the color fringing problem reducing the sharpness a little. Scanning in RGB does not do this averaging, so the individual channels will be better than the composite. This is all easily tested and verified on your personal scanner.

One caveat; when scanning in RGB mode, it is important that you make sure that the channel you are going to be keeping has not been clipped by the software. That can happen, and is especially a concern with pyro-style negatives where the three channels will have considerably different intensities.


---Michael

[sog1927]

Just a dumb question: If you have doubts about the transparency itself, why not check for fringing by looking directly at it with a good loupe? I've seen 10x and 12x Schneiders on *bay for a song lately (and bought them, by the way). I would think this amount of CA would be readily visible under that level of magnification, and wouldn't take more than a couple of seconds to evaluate.

Steve

The epson is a cheap scanner (in comparison to Creo flatbeds, drums scans, etc). Remember, the 4990 and 700 really were only designed for 4-6x enlargments (although some have argued up to 28x). Point is you have cropped way pass the 16x20 point, so of course your going to see some problems. Now the question is where is the problem? It is less likely the film, and more likely the scanner(cheap lens,guts) or your camera lens. Get a drum scan done, and if your drum scan improves, you know it is your scanner, if it doesn't improve you know it is the camera lens, and less likely the film.

[snaggs]

Guys.. this is simple. the lens in the V700 is not a APO lens.. it can't focus the RG & B wavelengths at the same distance.. There is a reason Nikon Coolscans have ED glass.

Daniel.

[Tim Lookingbill]

Yeah,

Flip through each RGB channel in PS and note the slight off alignment shift from one another in each gray channel. I get this on my 35mm scans off my refurbed Epson 4870.

snaggs' explanation makes since as to the cause of this.

[walter23]

That might be a cause, but the other possibility is an inability to perfectly align the information coming into the different R, G, and B scanner sites - I assume the design is similar to a digital camera, but with a linear array of pixel sites instead of a rectangular sensor area. Imperfect treatment of a parallax problem caused by this could lead to some offset in the colour channels.

Pure speculation though.