Ken Durling wrote:
> On Sat, 08 Sep 2001 09:45:55 -0400, you wrote:
>
> >Yes, I think you are correct. I learned a long time ago that to increase color
> >saturation on slides, set the ISO speed a bit higher than it actually is, ie: ISO
>200
> >slide film should be exposed as though is ISO 250. Then process it normally.
>
> Arrgg, it's 7am and you guys are confusing me. So why do people shoot
> Velvia (ISO 50) at ISO 40 to increase saturation??
They don't (even if they think that's what they're doing). They're compensating for
Velvia's
excessive contrast by overexposing slightly. Velvia is already so oversaturated that
the
overexposure can sometimes produce more realistic color, especially in sunny
conditions.
Velvia is a peculiar film, and there are a lot of different approaches to using it.
Some
folks use it for its sharpness and fine grain and live with the contrast and
hypersaturation. Some folks like its excesses, especially those who shoot for magazine
reproduction. Then there are those who are just confused, like the French pro I read
about
not long ago who meters at an ISO setting of 80 and then has it pushed one stop,
thereby
increasing contrast by underexposing and overdeveloping, and trying to compensate by
underexposing by slightly less than he's overdeveloping. Sheesh! Just use Provia 100F!
:-)
And then there are those photographers who seem to think ISO ratings are some kind of
marketing scam, and talk about the "true" speed of films based on nothing more than
what
they get from their own cameras--sometimes you see claims that Velvia is "really" ISO
40 (or
that Kodak E100 is "really" ISO 80, etc.). The actual ISO rating is derived from a set
of
specific repeatable tests under specific repeatable conditions. As such it represents
an
accurate measurement of film sensitivity *under those conditions* that allows different
films to be easily compared and exposure meters to be matched to the film in use.
There are
however all kinds of factors that can affect the actual performance of a film in the
field.
First of all there's variability in the accuracy of meters, as well as in how they're
used.
There are also factors introduced at the taking stage: the well-known 1/3-stop
underexposure of early examples of the EOS 3 or the less-well-known 1/3-stop
overexposure of
the Nikon F100, for example. Also, the apertures of some lenses aren't as accurate as
we
assume: for example, CdI recently tested the new Sigma 24-70/2.8 lens and found that
their
test sample consistently underexposed by 1/3 to 2/3 stop. The point being, what may
appear
to be an "incorrect" ISO rating for a film is almost certainly the result of a
combination
of equipment, technique, and personal preference. In other words, some photographers
prefer
the results they get when rating Velvia at EI 40 with their equipment and working
methods,
but that doesn't mean it's really an ISO 40 film. And remember too that any exposure
reading
is really just a guideline--to achieve some specific result you may need to deviate
from
what your meter tells you.
If you'd like to know what the specifics are for the testing of color slide film, you
can
order ISO 2240:1994 from the ISO website:
http://www.iso.ch/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=7050&ICS1=37&ICS2=40&ICS3=20
A complete list of ISO standards and procedures related to photographic film, paper,
and
cartridges can be found at:
http://www.iso.ch/iso/en/CatalogueListPage.CatalogueList?ICS1=37&ICS2=40&ICS3=20
As for increasing saturation in color negative film . . . I suppose it could be done by
varying exposure, but it's probably easier to do it in the printing stage, by choosing
a
paper with the saturation characteristics you want. And of course choosing the right
film in
the first place is helpful; print films seem to offer a wider range of saturation
levels
than do the current slide films (compare Portra NC with a high-saturation supermarket
film,
for example).
fcc
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