Digital BW, The Print

Steve Kale wrote:
>So I am now trying to understand filters a bit better.  As I understand it 
>a Red filter will
>allow red light to pass easily but will block (and hence darken on B&W 
>film) its
>complementary colour.  Is the "complementary colour" both blue and 
>green?  Hence, is
>stripping out the blue and green channels the loose equivalent of using a 
>strong red filter?
>What about a deep yellow filter No. 12?  Mix green and blue and one gets 
>yellow right?
>What is the complementary colour of yellow?


A filter will pass light of its own color and of the "adjacent" colors on a 
color wheel. It blocks some or almost all of the light from the other 
colors.  The darker (intensity) the filter, the more it blocks.

A sky is closer to cyan than blue. Red is opposite cyan, so it darkens 
skies the most.   Deep yellow will be more opposite from blue than cyan, so 
it won't darken the sky quite as much.  But still a fair amount.

Darkening an overcast sky is another matter entirely, since it is 
gray.  You have to use a neutral density filter there.

Also, when using a filter with film, you have to increase exposure because 
of the darkening effect of the filter.  This means you let more of the 
matching filter color in than if you had not increased the exposure.  That 
is why a filter will "lighten" objects of its color.

I would think you are on the right track regarding the color channels idea.

Instead of PS, I use Picture Window Pro, which allows the application of 
colored filters to the BW image, giving the effect you are looking 
for.   If PS doesn't do it, there must be plug-ins.

-Don M