Digital BW, The Print

scrber writes:

> After 2-3 years of digital darkrooming and B&W printing I am still 
> not happy with my methods to darken blue skies.

Take a snapshot, adjust curves to make the sky as dark as you require,
take another snapshot, switch back to the first snapshot, then use the
history brush or equivalent tool to brush the darkened sky back in.
This has to be done by hand, unless the transition line between sky and
the rest of the image is smooth and regular enough to allow you to apply
a mask.

> I am familiar with the red filter / polariser filters on film
> capture, but my efforts to successfully replicate this digitally 
> have never been particularly fruitful.

It cannot be replicated digitally, because the necessary information is
missing from the RGB image.  You need to use filters at the time of
image capture.  Red filters can be partially simulated, although it may
produce noise or posterization.  Polarization can only be simulated by
modifying the image pixel-by-pixel.

> I have tried many channel mixer, hue/sat methods to convert
> my digital colour images to B&W but all result in some sort
> of posterisation or noise in the sky.  I cannot get a smooth
> dark sky.

If the image has had less then 16 bits per channel at any time, you may
see posterization if you make large modifications to the image.  This
cannot be avoided.  You need adequate bit depth to avoid posterization.

> Selective darkening also is very difficult if you are
> really trying to go dark, the halos/  edges are almost
> impossible to avoid.

Nevertheless, that's the way it is done.  Sometimes, if there is a sharp
color or luminosity difference demarcating the transition to sky, you
can use other tools to speed up the process, but it's still mostly long
work by hand if you want the best results.

> Even after running neat image or selective bluring to remove sky 
> noise I end up with clear banding in the sky as it gets darker.

You may have to manually smear and clone to reduce banding.  It will be
hard to hide completely.

> Anyone want to share how you get those really dark / black skies
> with me, any real way of doing this from a digital colour file?

Image files don't contain enough information for many types of
filtering.  The best way is to use something like a polarizing filter at
the moment of capture.  If you are lucky, there may be shortcuts for a
given image, but very often it is a lot of hard work.

> I know this is slightly of topic but most of the 'retouching' or
> editing forums are populated by colour gurus, this is really the 
> knowledge base for B&W.

The principles are the same for both B&W and color, except that
obviously any manipulations based on color are impossible in B&W (as
opposed to merely difficult and often unsatisfactory in color), and
there may be more of a tendency to posterization in B&W because you have
only one channel instead of three (but if you work with 16-bit channels
and the original image is clean, this should not be a problem).
Isolating the sky with shortcuts is far harder in B&W, since intensity
is the only possible clue for transitions, and sometimes not even that.