On Tue, 27 Feb 2001 15:17:24 +1030 Mark Thomas (markthom@camtech.net.au)
wrote:
> Is this really true? I have encountered images where playing with RGB
> curves has just made me feel like I am drowning (perhaps just in my
> personal pool of insufficient knowledge!)
It's not easy, but it does get easier and quicker to do, with experience. 90%
of the problem is identifying what is wrong - what colour the error is and
which tones it affects. This *does* get hard if there are a compound mixture of
defects, eg wrong colour temp + crossed curves.
Frankly the best thing to do is to spend time playing with it. Deadlines help
concentrate the mind too:)
Often software provides different ways to control much the same thing, eg PS
adjust levels, adjust curves, adjust colour balance all provide a different UI
to the same fundamental parameter, gamma of the individual channels. The trick
is largely knowing which set of controls is appropriate and when. The right
ones make matters easier.
For instance : say you have a simple case of a pic being too cool because it
was taken in light which was too blue. You /could/ fix this entirely by
dragging curves around or setting the levels of each channel differently, but
it's a far simpler operation to use levels just to set levels (assume auto
levels does not help with this).
However it's much easier to use Adjust colour balance. My personal way of
dealing with this is to try and take out the most obvious error first, which
usually means increasing yellow/decreasing blue. However there is usually a
secondary shift which also needs dealing with by moving the cyan-red slider
toward red. It can be magenta-green though.
You should never use all 3 sliders, 2 max. If you feel tempted to, you're doing
something wrong with the others.
A curious thing I've noticed is that when colour has been successfully
corrected, the sliders always line up diagonally if there's a 2 channel error.
This is a useful shortcut guide once you are sure which channels need
correcting. Mid greys in the pic make life much easier here.
> If it really was that easy, why would we need colour filters (eg for
> tungsten lighting on daylight film)?
It can be done, though the error is rather gross with slide film because of
its contrasty nature. I have 'rescued' daylight Kodachrome exposed via tungsten
with no filter with no obvious ill effects. It's easy with colour neg, and a
main reason I have swapped to it + scanning is the refreshing ability to be
able to take colour pics in almost any lighting conditions without filters.
Regards
Tony Sleep
- Online portfolio & exhibit; + film scanner info &
comparisons
