I need help with Red Flower and D200

Started Jun 16, 2007 | Discussions thread
Tom Christiansen Senior Member • Posts: 2,239
How to fix your red flowers, part 2 of 2

(...continued...)

Kerry Pierce wrote:

IMO, the main thing is not to oversaturate the red channel.

Yes; or the blue, if a blue, a purple/magenta/pink, or a violet flower.

Then you can adjust the color balance easier.

You can, although I hate having to do that. Even with the right white
balance and exposure, for some violet-color blooms (not Nikon's
strongest suit), I find I have no choice but to nudge the curve a little
on the H dialogue in Nikon Capture over near the red/blue boundary.

I think you can also use filtration. Julia would likely know which
color filter would work best, depending on the shade of red you're
looking at.

You may be thinking of her advice to use color-correcting filters to
reduce noise in a channel that's underrepresented. White balancing is
going to boost (push) channels that may have very little data in them
anyway, which increases noise. This changes your effective ISO
sensitivity in those channels to something 2-3 stops worse than you
start with. If you start at ISO 100, it may not be so bad, but if you
start already pushed 2 stops to ISO 400, you do not want to push it
2-3 more stops past that!

It also risks blowing out highlights and/or reducing dynamic range.
Shooting in UniWB (and normal saturation and linear contrast) works to
see the true capture data in your histograms. Because of this, white
balance issues turn into exposure issues.

In incandescent light, you don't have enough blue, so you'd use
something like an 80A to boost it optically. In normal daylight,
you have too much green, so you'd use a magenta/purple (CC40M)
filter to pull up the the non-green. In my kind of daylight, an
open sky above 2 miles in altitude, you've got too much blue, so
you'd use a yellow filter.

Apparently this can clear up the so-called purple problem, since
daylight is so biased toward green but the purples are colors defined to
be having no green in them. It's otherwise hard to talk the Bayer
demosaïcking into ignoring green, since half its pixels are green data,
but you aren't supposed to have any (or much) of that.

Now that I poke around, it seems that exactly which density CC filter
you'll need will vary by body (D2H, D2X, D200). For example,

http://forums.dpreview.com/forums/read.asp?forum=1021&message=23121240

And that reminds me: on a D70, you really have to use a hot-mirror filter.

That's because, as on the D2H, the D70's IR-filter isn't strong enough.
It messes up your color scheme, subtly but substantially. The red
channel is obviously sensitive to near-IR, but it turns out the blue
channel is, too.

Shoot with a D70 some green foliage and red or orange flowers, and you
get big color shifts, and not just in the blooms: the green gets
yellower as the near-IR excites the red and blue channels.

When I shoot the D70, I always use use a B&W 486 IR+UV cut-filter, and
this brings its color accuracy up to the D200's standards as far as I
can see. Another important reason for the hot mirror is that otherwise
IR contamination will also trick your D70's AWB.

Hope this helps!

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tom

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