Wayne Culberson wrote:
>Fernando Gonzalez Gentile wrote:
>
> on 11/04/2005 16:04, Wayne Culberson at waynecul@xxxxxxxxxxxxxxx, wrote:
>
>>The 81a filter,
>>being slightly warm (orangeish-brownish-yellowish) corrects the color back
>>to nearer the standard at high altitudes.
>>
>>
> Yes, I have one of them, though never been to such a high altitude so find
> it useless. Personally, I would use a skylight 1a and a polarizer.
> I wouldn't like to see all my slides warmed by a 81a.... and if the
> atmosphere is bluish and shinny up there, why no capture it so ? You could
> warm them up w/PS later.
>
>Okay, I'll see if I can remember this correctly. If not, Moose, who seems
>to have a better memory than any elephant, and certainly better than me, can
>probably give the correct answer :-)................
>You won't notice the color shift problem so much with print film, as the
>one-hour developing machine will "correct" the color of the print, no matter
>what color the film saw.
>
>
There is an underlying problem with correction either in PS or an
automated printer that needs understanding if one wants first class results.
Film 'sees' much further into the ultra-violet than do our eyes. And it
all happens in only one of the color layers of the film. So when a
picture is taken at high altitude where there is a lot of UV, there is a
lot of extra exposure of the blue. The problem is that the dye layer
doesn't distinguish between the blues we can see and the UV we can't
see. So once the image is captured, there is no way to separate the two.
In an image editor or the color balancing done by an automated printer,
the blues can be toned down. But, and here's the hitch, all the ordinary
blues are desaturated along with the UV derived blues. So one ends up
with a compromise between an image with insufficient ordinary blue
content and one with excessive blue in the other colors.
One can do a fairly good job of getting a good pic in most instances,
but the color balance is never as good as if the UV weren't captured at
all in the first place. Many color filter effects can be duplicated in
photo editors. This is one that can't.
A couple of years ago, Wayne posted some images from Bolivia with
blue/purple casts, and we had this discussion. I also tried correcting
the color. I seem to recall one that had essentially no natural blue in
it that came out very well. Another image with a mountain side in shade
fairly near and a distant vista really couldn't be corrected as a whole.
With the vista and sky correct, the vegetation on the mountain was a
very strange color. With the vegetation correct, the distant vista was
off and the sky lost its blue. One could break the image up and
individually correct different parts, but that's not so much fun. And as
Wayne has pointed out, he does slide shows, so post correction is
impossible. (OK, not impossible, just expensive and a lot of work.)
Based on that discussion, Wayne got filters and last years shots were
much better.
For anything but slides to be projected, I think it's actually better to
over correct and have to boost blue a bit in processing. I'm sure there
are very expensive filters with fairly sharp cut-offs. the one we buy
for photography have rather gradual curves. Thus the Skylight and UV
filters really don't cut out much UV, in order not to affect color
balance at the much more common lower altitudes. They just don't do much
at high altitude.
I believe the same problems apply, at least to some extent, with digital
sensors, but I can't remember where I may have learned that. I suppose
it would be the case that the filters over the 'blue' pixels have broad
curves that let in UV.
Finally, one can't mount filters on an XA, but one may simply hold one
in front of the lens in the other hand. The location of the exposure
sensor is such that it is easily covered too. A little kludgy, but it
works when needed.
Moose
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