Colour fringing with V750?

[walter23]

Thanks Michael, I'll check that to see whether it's a taking lens CA issue or a scanning issue.

[Gene McCluney]

Fringing on a scanner can be because the transparency or negative is not being held completely rigid. It can also be due to multi-pass scanning which is normally done to get better shadow noise, but most scanners can't reposition the scanning head exactly for each scan, thus fringing.

[Michael Mutmansky]

Gene,

Color fringing is not caused by multipass scanning or film stability problems in my experience. It is related purely to the way the R, G, and B read the data on the original, and the fact that the three sensors cannot occupy the same physical location at the same time, and hence, they have a little bit of parallax error in the system.

I actually find it surprising that the typical scanner software doesn't do a mathematical shift to reduce this error. Maybe they do, but are unable to correct for the problem fully.

The problem you are describing in multipass scanning is a reduction in sharpness as multipass scanning occurs. This is very real, and is easily seen in a close comparison of single pass vs. multipass. However, it normally does not exhibit worse fringing when this happens. In fact, I think it could help obliterate the sharpness of the edges in the scan in a manner that could reduce the appearence of the color fringing.

We normally do a single- vs. multipass example at our workshops to help explain why we don't generally recommend multipass scanning unless there is a significant shadow noise problem in the image.


---Michael

[Chris Strobel]

I thought it was the blue channel, now I'm confused

I've seen other folks talk about copying B&W in RGB and the "dumping" all but the green channel to get the sharpest image. At what step is this done? IE in the scan set up or later in Photoshop?

[Kirk Gittings]

Chris it depends on the scanner actually. Pick the one that is the sharpest and the least noisy. I have seen it both on blue and green depending on the scanner. The red is always the worst.

[neil poulsen]

Others have minimized the problem by flipping the film over and scanning (then re-flipping in Photoshop). I have seen this issue on scans made from a 3170 all the way up to the V series.

Do you scan with the emulsion towards or away from the sensor? Are you suggesting that scanning with emulsion towards the scanner or away from the scanner will aleviate this problem?

[walter23]

Gene,

Color fringing is not caused by multipass scanning or film stability problems in my experience. It is related purely to the way the R, G, and B read the data on the original, and the fact that the three sensors cannot occupy the same physical location at the same time, and hence, they have a little bit of parallax error in the system.

I looked carefully at a B&W negative scan (scanned in colour transparency mode) and I would have to agree with this assessment. If you go to the channels list in photoshop and turn on single layers at a time you can see that the red channel is offset a bit from the green, and the green from the blue. It actually looked pretty uniform so I'd imagine if you cared enough about it you could write a script to perform an alignment in photoshop (e.g. with a slider to move the layers around by a couple of pixels, or just choose a set pixel value based on your measured distances). It looks like maybe a 2 to 4 pixel misalignment at 6400 DPI. It probably wouldn't be worth the effort for most printing efforts though.

Taking a single channel (any one) seems to give a bit more definition just by eliminating these other offset colour channels. But not much.

[walter23]

Here's an animation from a maximum resolution 6400 DPI scan. First frame is an RGB scan of the negative (highly saturated for effect, but only for the sake of this composite frame - the individual channels are from the original non-saturated scan), the next three are the R, G, and B channels. It's not a big deal at 2400 DPI or so I'd guess. I think the red is in fact the softest, the other two (blue, green) are about equal. They are in fact a bit offset though; interesting to know. Clever software should be able to correct this if it matters (which it probably doesn't).



This was scanned with silverfast in 48 bit colour mode without any sharpening applied. I think based on this that the best B&W scanning method would be to scan in colour with no sharpening applied, grab a channel, and then sharpen that.

For some context, the scan was from this (ugly test) negative (taken from a fraction, maybe 1/5th the area, of the little red frame):

[Ted Harris]

To follow on with Michael's post. When he and I did our first series of tests on a bunch of different scanners (some two years ago) we tested color fringing and it is a fact-of-life aross the board with all consumer scanners, some are a bit better than others but it is always there. In fact even the high end scanners exhibit a small amount of color fringing, but very very small compared to the consumer scanners.

[Kirk Gittings]

Nice demo Walter. I would also apply a steep curve (makes noise stand out) to the green and blue channel examples and see which has the least noise. You won't need to retest this scanner for awhile as they don't change much in this regard as they age.