J
Juha Koivisto
I noticed a strange problem with Minolta Scan Elite 5400 and VueScan
(8.0.7, Linux): the colors in the dark parts of a scanned slide have a
reddish cast in one end (top?) of the ccd, while the other end tends
toward green.
The effect is very subtle and would not be seen in most real scans, but
I did notice it in a photo with a lot of dark sky, which prompted me to
do some more testing... I put two test scans here temporarily:
<http://www.hut.fi/u/jkoivist/se5400/color.html>
Granted, the test is a bit artificial, as no-one would want to scan pure
black slides with colors boosted to the level of the first example, but
nevertheless, the problem is there and (like said) can sometimes be seen
in real photos as well.
The colors are not in the film - this is quite clear, but just to be
methodical about it, I flipped the film around and scanned it again,
with identical results.
The problem seems to affect only dark colours, and (as far as I could
tell with some quick testing) the Minolta software does not suffer from
it. It also seems that the colors may shift slightly more toward green
as the scanner warms up, but I'm not sure about this yet.
I have two theories about the cause: Either it's because of temperature-
dependent dark current in the ccd (after all, it's mounted vertically,
and heat rises up - but then why are red and green channels affected
differently?), or it could also be due to uneven lamp color which would
be properly compensated for in the "clear end" but the algorithm would
somehow overcompensate in the dark end.
The problem may be worse with negatives, as it would then affect bright
colors which are more visible. I don't have any photos with evenly
illuminated white background so I couldn't test it much (though I did
some scans of fully exposed film and the colors were there) - I'll try
to include some test shots on the next roll.
In any case the effect seems predictable enough that it should be easy
to fix. I'll be happy to do further testing if needed.
Could some other VueScan users also try scanning dark slide film? Do
you get similar results?
(8.0.7, Linux): the colors in the dark parts of a scanned slide have a
reddish cast in one end (top?) of the ccd, while the other end tends
toward green.
The effect is very subtle and would not be seen in most real scans, but
I did notice it in a photo with a lot of dark sky, which prompted me to
do some more testing... I put two test scans here temporarily:
<http://www.hut.fi/u/jkoivist/se5400/color.html>
Granted, the test is a bit artificial, as no-one would want to scan pure
black slides with colors boosted to the level of the first example, but
nevertheless, the problem is there and (like said) can sometimes be seen
in real photos as well.
The colors are not in the film - this is quite clear, but just to be
methodical about it, I flipped the film around and scanned it again,
with identical results.
The problem seems to affect only dark colours, and (as far as I could
tell with some quick testing) the Minolta software does not suffer from
it. It also seems that the colors may shift slightly more toward green
as the scanner warms up, but I'm not sure about this yet.
I have two theories about the cause: Either it's because of temperature-
dependent dark current in the ccd (after all, it's mounted vertically,
and heat rises up - but then why are red and green channels affected
differently?), or it could also be due to uneven lamp color which would
be properly compensated for in the "clear end" but the algorithm would
somehow overcompensate in the dark end.
The problem may be worse with negatives, as it would then affect bright
colors which are more visible. I don't have any photos with evenly
illuminated white background so I couldn't test it much (though I did
some scans of fully exposed film and the colors were there) - I'll try
to include some test shots on the next roll.
In any case the effect seems predictable enough that it should be easy
to fix. I'll be happy to do further testing if needed.
Could some other VueScan users also try scanning dark slide film? Do
you get similar results?