> There is no way with to achieve the effect of a sharp cut filter apart from
> using a sharp cut filter when you shoot. Adjusting RGB channels doesn't
> give the wavelength discrimination you can achieve with specific filters -
> like in the case of different greens that you mention. If you look at the
> colour sensitivity of a typical sensor (film or digital) there is
> considerable overlap of spectral sensitivity in the R-G & G-B ranges.
The best B&W conversion "trick" I've seen so far is with a variation
of the following. In your digital editor of choice, (PWP is especially
good at this) follow these steps:
1. Extract luminance channel to a new image or layer,
2. Convert to mono the the color image using the color filteration of
choice, OR extract the color channel of choice. For example, extract
Red only. Output to a new image or layer.
3. Combine the two new monochrome images or layers using "Hard Light".
Move the sliders a bit until the right contrast is achieved.
This is best done if you haven't gone to too much extreme in your
contrast settings beforehand. It's best to leave a little bit of space
on the histogram before you start the process. Otherwise, kiss your
highlights and shadows goodbye.
For those of us who like to have "balls to the wall" Red #25 effect,
this is the way to achieve it. Just pulling the red channel out gets
you part of the way, but doesn't give the same resulting curves or
densities as we've grown to love with classic Tri-X.
BTW, I keep referencing Tri-X. Tri-X has three specific aspects to the
color response. First of all, it tends towards the blue sensitivity.
If it was a color film, you would be sensing that it is white-balanced
for incandescent lighting. This is why most B&W films seem to work a
little better outdoors if you use a yellow filter. Secondly, Tri-X has
a little more green sensitivity. This gives more texture and tonal
contrast to skintones. Thirdly, Tri-X has a slightly truncated near-IR
response. As a result, it responds better to a red filter on blue
skies.
For us Ilford Dogs, classic PanF is usually a very good film for
landscape photography and responds to filters very well. Delta 100 is
fantastic for use with yellow and red filters, but definitely not good
for use with green or cyan filters. Delta 400 is actually quite
"adjustable", and can be configured to match Tri-X well. HP5 is Tri-X
without the subject isolation that Tri-X gives.
--
Ken Norton
ken@xxxxxxxxxxx
http://www.zone-10.com
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